CDaoDatabase Class

Represents a connection to an Access database using Data Access Objects (DAO).

Syntax

class CDaoDatabase : public CObject

Members

Public Constructors

Name	Description
CDaoDatabase::CDaoDatabase	Constructs a CDaoDatabase object. Call Open to connect the object to a database.

Public Methods

Name	Description
CDaoDatabase::CanTransact	Returns nonzero if the database supports transactions.
CDaoDatabase::CanUpdate	Returns nonzero if the CDaoDatabase object is updatable (not read-only).
CDaoDatabase::Close	Closes the database connection.
CDaoDatabase::Create	Creates the underlying DAO database object and initializes the CDaoDatabase object.
CDaoDatabase::CreateRelation	Defines a new relation among the tables in the database.
CDaoDatabase::DeleteQueryDef	Deletes a querydef object saved in the database's QueryDefs collection.
CDaoDatabase::DeleteRelation	Deletes an existing relation between tables in the database.
CDaoDatabase::DeleteTableDef	Deletes the definition of a table in the database. This deletes the actual table and all of its data.
CDaoDatabase::Execute	Executes an action query. Calling Execute for a query that returns results throws an exception.
CDaoDatabase::GetConnect	Returns the connection string used to connect the CDaoDatabase object to a database. Used for ODBC.
CDaoDatabase::GetName	Returns the name of the database currently in use.
CDaoDatabase::GetQueryDefCount	Returns the number of queries defined for the database.
CDaoDatabase::GetQueryDefInfo	Returns information about a specified query defined in the database.
CDaoDatabase::GetQueryTimeout	Returns the number of seconds after which database query operations time out. Affects all subsequent open, add new, update, and edit operations and other operations on ODBC data sources (only) such as Execute calls.
CDaoDatabase::GetRecordsAffected	Returns the number of records affected by the last update, edit, or add operation or by a call to Execute.
CDaoDatabase::GetRelationCount	Returns the number of relations defined between tables in the database.
CDaoDatabase::GetRelationInfo	Returns information about a specified relation defined between tables in the database.
CDaoDatabase::GetTableDefCount	Returns the number of tables defined in the database.
CDaoDatabase::GetTableDefInfo	Returns information about a specified table in the database.
CDaoDatabase::GetVersion	Returns the version of the database engine associated with the database.
CDaoDatabase::IsOpen	Returns nonzero if the CDaoDatabase object is currently connected to a database.
CDaoDatabase::Open	Establishes a connection to a database.
CDaoDatabase::SetQueryTimeout	Sets the number of seconds after which database query operations (on ODBC data sources only) time out. Affects all subsequent open, add new, update, and delete operations.

Public Data Members

Name	Description
CDaoDatabase::m_pDAODatabase	A pointer to the underlying DAO database object.
CDaoDatabase::m_pWorkspace	A pointer to the CDaoWorkspace object that contains the database and defines its transaction space.

Remarks

For information about the database formats supported, see the GetName member function. You can have one or more CDaoDatabase objects active at a time in a given "workspace," represented by a CDaoWorkspace object. The workspace maintains a collection of open database objects, called the Databases collection.

Usage

You can create database objects implicitly, when you create recordset objects. But you can also create database objects explicitly. To use an existing database explicitly with CDaoDatabase, do either of the following:

• Construct a CDaoDatabase object, passing a pointer to an open CDaoWorkspace object.

• Or construct a CDaoDatabase object without specifying the workspace (MFC creates a temporary workspace object).

To create a new Microsoft Jet (.MDB) database, construct a CDaoDatabase object and call its Create member function. Do not call Open after Create.

To open an existing database, construct a CDaoDatabase object and call its Open member function.

Any of these techniques appends the DAO database object to the workspace's Databases collection and opens a connection to the data. When you then construct CDaoRecordset, CDaoTableDef, or CDaoQueryDef objects for operating on the connected database, pass the constructors for these objects a pointer to your CDaoDatabase object. When you finish using the connection, call the Close member function and destroy the CDaoDatabase object. Close closes any recordsets you have not already closed.

Transactions

Database transaction processing is supplied at the workspace level—see the BeginTrans, CommitTrans, and Rollback member functions of class CDaoWorkspace.

ODBC Connections

The recommended way to work with Open Database Base Connectivity (ODBC) data sources is to attach external tables to a Microsoft Jet (.MDB) database.

Collections

Each database maintains its own collections of tabledef, querydef, recordset, and relation objects. Class CDaoDatabase supplies member functions for manipulating these objects.

Inheritance Hierarchy

CObject

CDaoDatabase

Requirements

Header: afxdao.h

CDaoDatabase::CanTransact

Call this member function to determine whether the database allows transactions.

BOOL CanTransact();

Return Value

Nonzero if the database supports transactions; otherwise 0.

Remarks

Transactions are managed in the database's workspace.

CDaoDatabase::CanUpdate

Call this member function to determine whether the CDaoDatabase object allows updates.

BOOL CanUpdate();

Return Value

Nonzero if the CDaoDatabase object allows updates; otherwise 0, indicating either that you passed TRUE in bReadOnly when you opened the CDaoDatabase object or that the database itself is read-only. See the Open member function.

Remarks

For information about database updatability, see the topic "Updatable Property" in DAO Help.

CDaoDatabase::CDaoDatabase

Constructs a CDaoDatabase object.

CDaoDatabase(CDaoWorkspace* pWorkspace = NULL);

Parameters

pWorkspace
A pointer to the CDaoWorkspace object that will contain the new database object. If you accept the default value of NULL, the constructor creates a temporary CDaoWorkspace object that uses the default DAO workspace. You can get a pointer to the workspace object via the m_pWorkspace data member.

Remarks

After constructing the object, if you are creating a new Microsoft Jet (.MDB) database, call the object's Create member function. If you are, instead, opening an existing database, call the object's Open member function.

When you finish with the object, you should call its Close member function and then destroy the CDaoDatabase object.

You might find it convenient to embed the CDaoDatabase object in your document class.

CDaoDatabase::Close

Call this member function to disconnect from a database and close any open recordsets, tabledefs, and querydefs associated with the database.

virtual void Close();

Remarks

It's good practice to close these objects yourself before you call this member function. Closing a CDaoDatabase object removes it from the Databases collection in the associated workspace. Because Close doesn't destroy the CDaoDatabase object, you can reuse the object by opening the same database or a different database.

If the database object is defined outside the scope of a function, and you exit the function without closing it, the database object will remain open until explicitly closed or the module in which it's defined is out of scope.

CDaoDatabase::Create

To create a new Microsoft Jet (.MDB) database, call this member function after you construct a CDaoDatabase object.

virtual void Create(
    LPCTSTR lpszName,
    LPCTSTR lpszLocale = dbLangGeneral,
    int dwOptions = 0);

Parameters

lpszName
A string expression that is the name of the database file that you are creating. It can be the full path and filename, such as "C:\\MYDB.MDB". You must supply a name. If you don't supply a filename extension, .MDB is appended. If your network supports the uniform naming convention (UNC), you can also specify a network path, such as "\\\\MYSERVER\\MYSHARE\\MYDIR\\MYDB". Only Microsoft Jet (.MDB) database files can be created using this member function. (Double backslashes are required in string literals because "\" is the C++ escape character.)

lpszLocale
A string expression used to specify collating order for creating the database. The default value is dbLangGeneral. Possible values are:

• dbLangGeneral English, German, French, Portuguese, Italian, and Modern Spanish

• dbLangArabic Arabic

• dbLangCyrillic Russian

• dbLangCzech Czech

• dbLangDutch Dutch

• dbLangGreek Greek

• dbLangHebrew Hebrew

• dbLangHungarian Hungarian

• dbLangIcelandic Icelandic

• dbLangNordic Nordic languages (Microsoft Jet database engine version 1.0 only)

• dbLangNorwdan Norwegian and Danish

• dbLangPolish Polish

• dbLangSpanish Traditional Spanish

• dbLangSwedfin Swedish and Finnish

• dbLangTurkish Turkish

dwOptions
An integer that indicates one or more options. Possible values are:

• dbEncrypt Create an encrypted database.

• dbVersion10 Create a database with Microsoft Jet database version 1.0.

• dbVersion11 Create a database with Microsoft Jet database version 1.1.

• dbVersion20 Create a database with Microsoft Jet database version 2.0.

• dbVersion30 Create a database with Microsoft Jet database version 3.0.

If you omit the encryption constant, an unencrypted database is created. You can specify only one version constant. If you omit a version constant, a database that uses the Microsoft Jet database version 3.0 is created.

Remarks

Create creates the database file and the underlying DAO database object and initializes the C++ object. The object is appended to the associated workspace's Databases collection. The database object is in an open state; don't call Open* after Create.

CDaoDatabase::CreateRelation

Call this member function to establish a relation between one or more fields in a primary table in the database and one or more fields in a foreign table (another table in the database).

void CreateRelation(
    LPCTSTR lpszName,
    LPCTSTR lpszTable,
    LPCTSTR lpszForeignTable,
    long lAttributes,
    LPCTSTR lpszField,
    LPCTSTR lpszForeignField);

void CreateRelation(CDaoRelationInfo& relinfo);

Parameters

lpszName
The unique name of the relation object. The name must start with a letter and can contain a maximum of 40 characters. It can include numbers and underscore characters but cannot include punctuation or spaces.

lpszTable
The name of the primary table in the relation. If the table doesn't exist, MFC throws an exception of type CDaoException.

lpszForeignTable
The name of the foreign table in the relation. If the table doesn't exist, MFC throws an exception of type CDaoException.

lAttributes
A long value that contains information about the relationship type. You can use this value to enforce referential integrity, among other things. You can use the bitwise-OR operator (|) to combine any of the following values (as long as the combination makes sense):

• dbRelationUnique Relationship is one-to-one.

• dbRelationDontEnforce Relationship is not enforced (no referential integrity).

• dbRelationInherited Relationship exists in a noncurrent database that contains the two attached tables.

• dbRelationUpdateCascade Updates will cascade (for more on cascades, see Remarks).

• dbRelationDeleteCascade Deletions will cascade.

lpszField
A pointer to a null-terminated string containing the name of a field in the primary table (named by lpszTable).

lpszForeignField
A pointer to a null-terminated string containing the name of a field in the foreign table (named by lpszForeignTable).

relinfo
A reference to a CDaoRelationInfo object that contains information about the relation you want to create.

Remarks

The relationship cannot involve a query or an attached table from an external database.

Use the first version of the function when the relation involves one field in each of the two tables. Use the second version when the relation involves multiple fields. The maximum number of fields in a relation is 14.

This action creates an underlying DAO relation object, but this is an MFC implementation detail since MFC's encapsulation of relation objects is contained within class CDaoDatabase. MFC doesn't supply a class for relations.

If you set the relation object's attributes to activate cascade operations, the database engine automatically updates or deletes records in one or more other tables when changes are made to related primary key tables.

For example, suppose you establish a cascade delete relationship between a Customers table and an Orders table. When you delete records from the Customers table, records in the Orders table related to that customer are also deleted. In addition, if you establish cascade delete relationships between the Orders table and other tables, records from those tables are automatically deleted when you delete records from the Customers table.

For related information, see the topic "CreateRelation Method" in DAO Help.

CDaoDatabase::DeleteQueryDef

Call this member function to delete the specified querydef—saved query—from the CDaoDatabase object's QueryDefs collection.

void DeleteQueryDef(LPCTSTR lpszName);

Parameters

lpszName
The name of the saved query to delete.

Remarks

Afterwards, that query is no longer defined in the database.

For information about creating querydef objects, see class CDaoQueryDef. A querydef object becomes associated with a particular CDaoDatabase object when you construct the CDaoQueryDef object, passing it a pointer to the database object.

CDaoDatabase::DeleteRelation

Call this member function to delete an existing relation from the database object's Relations collection.

void DeleteRelation(LPCTSTR lpszName);

Parameters

lpszName
The name of the relation to delete.

Remarks

Afterwards, the relation no longer exists.

For related information, see the topic "Delete Method" in DAO Help.

CDaoDatabase::DeleteTableDef

Call this member function to delete the specified table and all of its data from the CDaoDatabase object's TableDefs collection.

void DeleteTableDef(LPCTSTR lpszName);

Parameters

lpszName
The name of the tabledef to delete.

Remarks

Afterwards, that table is no longer defined in the database.

For information about creating tabledef objects, see class CDaoTableDef. A tabledef object becomes associated with a particular CDaoDatabase object when you construct the CDaoTableDef object, passing it a pointer to the database object.

For related information, see the topic "Delete Method" in DAO Help.

CDaoDatabase::Execute

Call this member function to run an action query or execute a SQL statement on the database.

void Execute(
    LPCTSTR lpszSQL,
    int nOptions = dbFailOnError);

Parameters

lpszSQL
Pointer to a null-terminated string containing a valid SQL command to execute.

nOptions
An integer that specifies options relating to the integrity of the query. You can use the bitwise-OR operator (|) to combine any of the following constants provided the combination makes sense. For example, you would not combine dbInconsistent with dbConsistent:

• dbDenyWrite Deny write permission to other users.

• dbInconsistent (Default) Inconsistent updates.

• dbConsistent Consistent updates.

• dbSQLPassThrough SQL pass-through. Causes the SQL statement to be passed to an ODBC data source for processing.

• dbFailOnError Roll back updates if an error occurs.

• dbSeeChanges Generate a run-time error if another user is changing data you are editing.

Remarks

Execute works only for action queries or SQL pass-through queries that don't return results. It doesn't work for select queries, which return records.

For a definition and information about action queries, see the topics "Action Query" and "Execute Method" in DAO Help.

Call the GetRecordsAffected member function of the database object to determine the number of records affected by the most recent Execute call. For example, GetRecordsAffected returns information about the number of records deleted, updated, or inserted when executing an action query. The count returned doesn't reflect changes in related tables when cascade updates or deletes are in effect.

Execute doesn't return a recordset. Using Execute on a query that selects records causes MFC to throw an exception of type CDaoException. (There is no ExecuteSQL member function analogous to CDatabase::ExecuteSQL.)

CDaoDatabase::GetConnect

Call this member function to retrieve the connection string used to connect the CDaoDatabase object to an ODBC or ISAM database.

CString GetConnect();

Return Value

The connection string if Open has been called successfully on an ODBC data source; otherwise, an empty string. For a Microsoft Jet (.MDB) database, the string is always empty unless you set it for use with the dbSQLPassThrough option used with the Execute member function or used in opening a recordset.

Remarks

The string provides information about the source of an open database or a database used in a pass-through query. The connection string is composed of a database type specifier and zero or more parameters separated by semicolons.

See the Open member function for a description of how the connection string is created. Once the connection string has been set in the Open call, you can later use it to check the setting to determine the type, path, user ID, Password, or ODBC data source of the database.

CDaoDatabase::GetName

Call this member function to retrieve the name of the currently open database, which is the name of an existing database file or the name of a registered ODBC data source.

CString GetName();

Return Value

The full path and file name of the database if successful; otherwise, an empty CString.

Remarks

If your network supports the uniform naming convention (UNC), you can also specify a network path—for example, "\\\\MYSERVER\\MYSHARE\\MYDIR\\MYDB.MDB". (Double backslashes are required in string literals because "\" is the C++ escape character.)

You might, for example, want to display this name in a heading. If an error occurs while the name is being retrieved, MFC throws an exception of type CDaoException.

The database type is indicated by the file or directory that the path points to, as follows:

Pathname points to..	Database type
.MDB file	Microsoft Jet database (Microsoft Access)
Directory that contains .DBF file(s)	dBASE database
Directory that contains .XLS file	Microsoft Excel database
Directory that contains .PDX file(s)	Paradox database
Directory that contains appropriately formatted text database files	Text format database

For ODBC databases such as SQL Server and Oracle, the database's connection string identifies a data source name (DSN) that's registered by ODBC.

CDaoDatabase::GetQueryDefCount

Call this member function to retrieve the number of queries defined in the database's QueryDefs collection.

short GetQueryDefCount();

Return Value

The number of queries defined in the database.

Remarks

GetQueryDefCount is useful if you need to loop through all querydefs in the QueryDefs collection. To obtain information about a given query in the collection, see GetQueryDefInfo.

CDaoDatabase::GetQueryDefInfo

Call this member function to obtain various kinds of information about a query defined in the database.

void GetQueryDefInfo(
    int nIndex,
    CDaoQueryDefInfo& querydefinfo,
    DWORD dwInfoOptions = AFX_DAO_PRIMARY_INFO);

void GetQueryDefInfo(
    LPCTSTR lpszName,
    CDaoQueryDefInfo& querydefinfo,
    DWORD dwInfoOptions = AFX_DAO_PRIMARY_INFO);

Parameters

nIndex
The index of the predefined query in the database's QueryDefs collection, for lookup by index.

querydefinfo
A reference to a CDaoQueryDefInfo object that returns the information requested.

dwInfoOptions
Options that specify which information about the recordset to retrieve. The available options are listed here along with what they cause the function to return about the recordset:

• AFX_DAO_PRIMARY_INFO (Default) Name, Type

• AFX_DAO_SECONDARY_INFO Primary information plus: Date Created, Date of Last Update, Returns Records, Updatable

• AFX_DAO_ALL_INFO Primary and secondary information plus: SQL, Connect, ODBCTimeout

lpszName
A string containing the name of a query defined in the database, for lookup by name.

Remarks

Two versions of the function are supplied so you can select a query either by index in the database's QueryDefs collection or by the name of the query.

For a description of the information returned in querydefinfo, see the CDaoQueryDefInfo structure. This structure has members that correspond to the items of information listed above in the description of dwInfoOptions. If you request one level of information, you get any prior levels of information as well.

CDaoDatabase::GetQueryTimeout

Call this member function to retrieve the current number of seconds to allow before subsequent operations on the connected database are timed out.

short GetQueryTimeout();

Return Value

A short integer containing the timeout value in seconds.

Remarks

An operation might time out due to network access problems, excessive query processing time, and so on. While the setting is in effect, it affects all open, add new, update, and delete operations on any recordsets associated with this CDaoDatabase object. You can change the current timeout setting by calling SetQueryTimeout. Changing the query timeout value for a recordset after opening doesn't change the value for the recordset. For example, subsequent Move operations don't use the new value. The default value is initially set when the database engine is initialized.

The default value for query timeouts is taken from the Windows registry. If there is no registry setting, the default is 60 seconds. Not all databases support the ability to set a query timeout value. If you set a query timeout value of 0, no timeout occurs; and communication with the database may stop responding. This behavior may be useful during development. If the call fails, MFC throws an exception of type CDaoException.

For related information, see the topic "QueryTimeout Property" in DAO Help.

CDaoDatabase::GetRecordsAffected

Call this member function to determine the number of records affected by the most recent call of the Execute member function.

long GetRecordsAffected();

Return Value

A long integer containing the number of records affected.

Remarks

The value returned includes the number of records deleted, updated, or inserted by an action query run with Execute. The count returned doesn't reflect changes in related tables when cascade updates or deletes are in effect.

For related information, see the topic "RecordsAffected Property" in DAO Help.

CDaoDatabase::GetRelationCount

Call this member function to obtain the number of relations defined between tables in the database.

short GetRelationCount();

Return Value

The number of relations defined between tables in the database.

Remarks

GetRelationCount is useful if you need to loop through all defined relations in the database's Relations collection. To obtain information about a given relation in the collection, see GetRelationInfo.

To illustrate the concept of a relation, consider a Suppliers table and a Products table, which might have a one-to-many relationship. In this relationship, one supplier can supply more than one product. Other relations are one-to-one and many-to-many.

CDaoDatabase::GetRelationInfo

Call this member function to obtain information about a specified relation in the database's Relations collection.

void GetRelationInfo(
    int nIndex,
    CDaoRelationInfo& relinfo,
    DWORD dwInfoOptions = AFX_DAO_PRIMARY_INFO);

void GetRelationInfo(
    LPCTSTR lpszName,
    CDaoRelationInfo& relinfo,
    DWORD dwInfoOptions = AFX_DAO_PRIMARY_INFO);

Parameters

nIndex
The index of the relation object in the database's Relations collection, for lookup by index.

relinfo
A reference to a CDaoRelationInfo object that returns the information requested.

dwInfoOptions
Options that specify which information about the relation to retrieve. The available options are listed here along with what they cause the function to return about the relation:

• AFX_DAO_PRIMARY_INFO (Default) Name, Table, Foreign Table

• AFX_DAO_SECONDARY_INFO Attributes, Field Information

The Field Information is a CDaoRelationFieldInfo object containing the fields from the primary table involved in the relation.

lpszName
A string containing the name of the relation object, for lookup by name.

Remarks

Two versions of this function provide access either by index or by name. For a description of the information returned in relinfo, see the CDaoRelationInfo structure. This structure has members that correspond to the items of information listed above in the description of dwInfoOptions. If you request information at one level, you also get information at any prior levels as well.

CDaoDatabase::GetTableDefCount

Call this member function to retrieve the number of tables defined in the database.

short GetTableDefCount();

Return Value

The number of tabledefs defined in the database.

Remarks

GetTableDefCount is useful if you need to loop through all tabledefs in the database's TableDefs collection. To obtain information about a given table in the collection, see GetTableDefInfo.

CDaoDatabase::GetTableDefInfo

Call this member function to obtain various kinds of information about a table defined in the database.

void GetTableDefInfo(
    int nIndex,
    CDaoTableDefInfo& tabledefinfo,
    DWORD dwInfoOptions = AFX_DAO_PRIMARY_INFO);

void GetTableDefInfo(
    LPCTSTR lpszName,
    CDaoTableDefInfo& tabledefinfo,
    DWORD dwInfoOptions = AFX_DAO_PRIMARY_INFO);

Parameters

nIndex
The index of the tabledef object in the database's TableDefs collection, for lookup by index.

tabledefinfo
A reference to a CDaoTableDefInfo object that returns the information requested.

dwInfoOptions
Options that specify which information about the table to retrieve. The available options are listed here along with what they cause the function to return about the relation:

• AFX_DAO_PRIMARY_INFO (Default) Name, Updatable, Attributes

• AFX_DAO_SECONDARY_INFO Primary information plus: Date Created, Date Last Updated, Source Table Name, Connect

• AFX_DAO_ALL_INFO Primary and secondary information plus: Validation Rule, Validation Text, Record Count

lpszName
The name of the tabledef object, for lookup by name.

Remarks

Two versions of the function are supplied so you can select a table either by index in the database's TableDefs collection or by the name of the table.

For a description of the information returned in tabledefinfo, see the CDaoTableDefInfo structure. This structure has members that correspond to the items of information listed above in the description of dwInfoOptions. If you request information at one level, you get information for any prior levels as well.

CDaoDatabase::GetVersion

Call this member function to determine the version of the Microsoft Jet database file.

CString GetVersion();

Return Value

A CString that indicates the version of the database file associated with the object.

Remarks

The value returned represents the version number in the form "major.minor"; for example, "3.0". The product version number (for example, 3.0) consists of the version number (3), a period, and the release number (0). The versions to date are 1.0, 1.1, 2.0, and 3.0.

For related information, see the topic "Version Property" in DAO Help.

CDaoDatabase::IsOpen

Call this member function to determine whether the CDaoDatabase object is currently open on a database.

BOOL IsOpen() const;

Return Value

Nonzero if the CDaoDatabase object is currently open; otherwise 0.

Remarks

CDaoDatabase::m_pDAODatabase

Contains a pointer to the OLE interface for the DAO database object underlying the CDaoDatabase object.

Remarks

Use this pointer if you need to access the DAO interface directly.

For information about calling DAO directly, see Technical Note 54.

CDaoDatabase::m_pWorkspace

Contains a pointer to the CDaoWorkspace object that contains the database object.

Remarks

Use this pointer if you need to access the workspace directly. For example, to obtain pointers to other database objects in the workspace's Databases collection.

CDaoDatabase::Open

You must call this member function to initialize a newly constructed CDaoDatabase object that represents an existing database.

virtual void Open(
    LPCTSTR lpszName,
    BOOL bExclusive = FALSE,
    BOOL bReadOnly = FALSE,
    LPCTSTR lpszConnect = _T(""));

Parameters

lpszName
A string expression that is the name of an existing Microsoft Jet (.MDB) database file. If the filename has an extension, it's required. If your network supports the uniform naming convention (UNC), you can also specify a network path, such as "\\\\MYSERVER\\MYSHARE\\MYDIR\\MYDB.MDB". (Double backslashes are required in string literals because "\" is the C++ escape character.)

Some considerations apply when using lpszName. If it:

• Refers to a database that is already open for exclusive access by another user, MFC throws an exception of type CDaoException. Trap that exception to let your user know that the database is unavailable.

• Is an empty string ("") and lpszConnect is "ODBC;", a dialog box listing all registered ODBC data source names is displayed so the user can select a database. You should avoid direct connections to ODBC data sources; use an attached table instead.

• Otherwise doesn't refer to an existing database or valid ODBC data source name, MFC throws an exception of type CDaoException.

bExclusive
A Boolean value that is TRUE if the database is to be opened for exclusive (nonshared) access and FALSE if the database is to be opened for shared access. If you omit this argument, the database is opened for shared access.

bReadOnly
A Boolean value that is TRUE if the database is to be opened for read-only access and FALSE if the database is to be opened for read/write access. If you omit this argument, the database is opened for read/write access. All dependent recordsets inherit this attribute.

lpszConnect
A string expression used for opening the database. This string constitutes the ODBC connect arguments. You must supply the exclusive and read-only arguments to supply a source string. If the database is a Microsoft Jet database (.MDB), this string is empty (""). The syntax for the default value—_T("")—provides portability for Unicode as well as ANSI builds of your application.

Remarks

Open associates the database with the underlying DAO object. You cannot use the database object to construct recordset, tabledef, or querydef objects until it's initialized. Open appends the database object to the associated workspace's Databases collection.

Use the parameters as follows:

• If you are opening a Microsoft Jet (.MDB) database, use the lpszName parameter and pass an empty string for the lpszConnect parameter or pass a password string of the form ";PWD=password" if the database is password-protected (.MDB databases only).

• If you are opening an ODBC data source, pass a valid ODBC connection string in lpszConnect and an empty string in lpszName.

For related information, see the topic "OpenDatabase Method" in DAO Help.

It's possible for a connection attempt to time out if, for example, the DBMS host is unavailable. If the connection attempt fails, Open throws an exception of type CDaoException.

The remaining remarks apply only to ODBC databases:

If the database is an ODBC database and the parameters in your Open call don't contain enough information to make the connection, the ODBC driver opens a dialog box to obtain the necessary information from the user. When you call Open, your connection string, lpszConnect, is stored privately and is available by calling the GetConnect member function.

If you wish, you can open your own dialog box before you call Open to get information from the user, such as a password, then add that information to the connection string you pass to Open. Or you might want to save the connection string you pass (perhaps in the Windows registry) so you can reuse it the next time your application calls Open on a CDaoDatabase object.

You can also use the connection string for multiple levels of login authorization (each for a different CDaoDatabase object) or to convey other database-specific information.

CDaoDatabase::SetQueryTimeout

Call this member function to override the default number of seconds to allow before subsequent operations on the connected database time out.

void SetQueryTimeout(short nSeconds);

Parameters

nSeconds
The number of seconds to allow before a query attempt times out.

Remarks

An operation might time out because of network access problems, excessive query processing time, and so on. Call SetQueryTimeout before opening your recordset or before calling the recordset's AddNew, Update, or Delete member functions if you want to change the query timeout value. The setting affects all subsequent Open, AddNew, Update, and Delete calls to any recordsets associated with this CDaoDatabase object. Changing the query timeout value for a recordset after opening doesn't change the value for the recordset. For example, subsequent Move operations don't use the new value.

The default value for query timeouts is 60 seconds. Not all databases support the ability to set a query timeout value. If you set a query timeout value of 0, no timeout occurs; the communication with the database may stop responding. This behavior may be useful during development.

For related information, see the topic "QueryTimeout Property" in DAO Help.

See also

CObject Class
Hierarchy Chart
CDaoWorkspace Class
CDaoRecordset Class
CDaoTableDef Class
CDaoQueryDef Class
CDatabase Class
CDaoException Class