CONSTRAINT Clause

Description

A constraint is similar to an index, although it can also be used to establish a relationship with another table.

You use the CONSTRAINT clause in ALTER TABLE and CREATE TABLE statements to create or delete constraints. There are two types of CONSTRAINT clauses: one for creating a constraint on a single field and one for creating a constraint on more than one field.

Note The Microsoft Jet database engine doesn't support the use of CONSTRAINT, or any of the data definition language (DDL) statements, with non-Microsoft Jet databases. Use the DAO Create methods instead.

Syntax

Single-field constraint:

CONSTRAINT name {PRIMARY KEY | UNIQUE | NOT NULL |
REFERENCES foreigntable [(foreignfield1, foreignfield2)]}

Multiple-field constraint:

CONSTRAINT name
{PRIMARY KEY (primary1[, primary2 [, ...]]) |
UNIQUE (unique1[, unique2 [, ...]]) |
NOT NULL (notnull1[, notnull2 [, ...]]) |
FOREIGN KEY (ref1[, ref2 [, ...]])
REFERENCES foreigntable [(foreignfield1 [, foreignfield2 [, ...]])]}

The CONSTRAINT clause has these parts

Part	Description

name	The name of the constraint to be created.
primary1, primary2	The name of the field or fields to be designated the primary key.
unique1, unique2	The name of the field or fields to be designated as a unique key.
notnull1, notnull2	The name of the field or fields that are restricted to non-Null values.
ref1, ref2	The name of a foreign key field or fields that refer to fields in another table.
foreigntable	The name of the foreign table containing the field or fields specified by foreignfield.
foreignfield1, foreignfield2	The name of the field or fields in foreigntable specified by ref1, ref2. You can omit this clause if the referenced field is the primary key of foreigntable.

Remarks

You use the syntax for a single-field constraint in the field-definition clause of an ALTER TABLE or CREATE TABLE statement immediately following the specification of the field's data type.

You use the syntax for a multiple-field constraint whenever you use the reserved word CONSTRAINT outside a field-definition clause in an ALTER TABLE or CREATE TABLE statement.

Using CONSTRAINT, you can designate a field as one of the following types of constraints:

You can use the UNIQUE reserved word to designate a field as a unique key. This means that no two records in the table can have the same value in this field. You can constrain any field or list of fields as unique. If a multiple-field constraint is designated as a unique key, the combined values of all fields in the index must be unique, even if two or more records have the same value in just one of the fields.
You can use the PRIMARY KEY reserved words to designate one field or set of fields in a table as a primary key. All values in the primary key must be unique and not Null, and there can be only one primary key for a table. Note Don't set a PRIMARY KEY constraint on a table that already has a primary key; if you do, an error occurs.
You can use the FOREIGN KEY reserved words to designate a field as a foreign key. If the foreign table's primary key consists of more than one field, you must use a multiple-field constraint definition, listing all of the referencing fields, the name of the foreign table, and the names of the referenced fields in the foreign table in the same order that the referencing fields are listed. If the referenced field or fields are the foreign table's primary key, you don't have to specify the referenced fields — by default, the database engine behaves as if the foreign table's primary key is the referenced fields.

See Also

ALTER TABLE statement, CREATE INDEX statement, CREATE TABLE statement, CreateField method ("DAO Language Reference"), CreateIndex method ("DAO Language Reference"), CreateRelation method ("DAO Language Reference"), CreateTableDef method ("DAO Language Reference"), DROP statement.

Example

This example creates a new table called ThisTable with two Text fields.

Sub CreateTableX1()

    Dim dbs As Database

    ' Modify this line to include the path to Northwind
    ' on your computer.
    Set dbs = OpenDatabase("Northwind.mdb")

' Create a table with two text fields.
    dbs.Execute "CREATE TABLE ThisTable " _
        & "(FirstName TEXT, LastName TEXT);"

    dbs.Close

End Sub

This example creates a new table called MyTable with two Text fields, a Date/Time field, and a unique index made up of all three fields.

Sub CreateTableX2()

    Dim dbs As Database

    ' Modify this line to include the path to Northwind
    ' on your computer.
    Set dbs = OpenDatabase("Northwind.mdb")

    ' Create a table with three fields and a unique
    ' index made up of all three fields.
    dbs.Execute "CREATE TABLE MyTable " _
        & "(FirstName TEXT, LastName TEXT, " _
        & "DateOfBirth DATETIME, " _
        & "CONSTRAINT MyTableConstraint UNIQUE " _
        & "(FirstName, LastName, DateOfBirth));"

    dbs.Close

End Sub

This example creates a new table with two Text fields and an Integer field. The SSN field is the primary key.

Sub CreateTableX3()

     Dim dbs As Database

    ' Modify this line to include the path to Northwind
    ' on your computer.
    Set dbs = OpenDatabase("Northwind.mdb")

    ' Create a table with three fields and a primary
    ' key.
    dbs.Execute "CREATE TABLE NewTable " _
        & "(FirstName TEXT, LastName TEXT, " _
        & "SSN INTEGER CONSTRAINT MyFieldConstraint " _
        & "PRIMARY KEY);"

    dbs.Close

End Sub

Example (Microsoft Access)

To try the following examples in Microsoft Access, first create a new query in the Northwind sample database. Close the Show Table dialog box without specifying a table or query. Switch to SQL view, paste an individual example into the SQL window, and run the query.

Warning These examples makes changes to the Northwind sample database. Before beginning, you may wish to make a backup copy of the sample database.

The following example creates a new table with two Text fields:

CREATE TABLE FirstTable (FirstName TEXT, LastName TEXT);

The next example creates a new table with two Text fields, a Date/Time field, and a unique index made up of all three fields:

CREATE TABLE SecondTable (FirstName TEXT,
LastName TEXT, DateOfBirth DATETIME,
CONSTRAINT MyTableConstraint UNIQUE (FirstName, LastName, DateOfBirth));

The following example creates a new table with two Text fields and an Integer Number field. The SSN field is the primary key.

CREATE TABLE ThirdTable (FirstName TEXT, LastName TEXT, SSN INTEGER
CONSTRAINT MyFieldConstraint PRIMARY KEY);