Note:As with other constraints, naming aFOREIGN KEYconstraint usingCONSTRAINT constraint_nameis optional. But doing so makes it easier to make changes to or delete the constraint. This is especially helpful when defining multiple constraints. Recommended Readings:
PostgreSQL foreign key constraint syntax The following illustrates a foreign key constraint syntax: [CONSTRAINT fk_name] FOREIGN KEY(fk_columns) REFERENCES parent_table(parent_key_columns) [ON DELETE delete_action] [ON UPDATE update_action] In this syntax: First, specify the name for the foreign...
PRIMARYKEY(OrderID), FOREIGNKEY(PersonID)REFERENCESPersons(PersonID) ); To allow naming of aFOREIGN KEYconstraint, and for defining aFOREIGN KEYconstraint on multiple columns, use the following SQL syntax: CREATETABLEOrders ( OrderID intNOTNULL, ...
Oracle foreign key constraint syntax Oracle allows you to create, add, drop, disable, and enable a foreign key constraint. Create a foreign key constraint The following statement illustrates the syntax of creating a foreign key constraint when you create a table: CREATE TABLE child_table ( ....
InnoDBsupports foreign key constraints. The syntax for a foreign key constraint definition inInnoDBlooks like this: [CONSTRAINT [symbol]] FOREIGN KEY [index_name] (index_col_name, ...) REFERENCEStbl_name(index_col_name,...) [ON DELETEreference_option] ...
A foreign key relationship involves a parent table that holds the initial column values, and a child table with column values that reference the parent column values. A foreign key constraint is defined on the child table. The essential syntax for a defining a foreign key constraint in aCREATE...
SyntaxForeign key constraints can be defined at the table level. However, if you only want the constraint to apply to a single column, it can be applied at the column level.Note: You can also add the FOREIGN KEY constraint to existing tables through ADD CONSTRAINT. ...
The foreign key is defined in the child table. The syntax is as follows: ForeignKeyDef CONSTRAINTIdentifierFOREIGNKEYIdentifier(ColumnName,)REFERENCESTableName(ColumnName,)ONDELETEReferenceOptionONUPDATEReferenceOption ReferenceOption RESTRICTCASCADESETNULLSETDEFAULTNOACTION ...
You can drop a foreign key constraint using the followingALTER TABLEsyntax: ALTER TABLEtbl_nameDROP FOREIGN KEYfk_symbol; If theFOREIGN KEYclause defined aCONSTRAINTname when you created the constraint, you can refer to that name to drop the foreign key constraint. Otherwise, a constraint name ...
Syntax Foreign Key constraints can be defined at thetable level. However, if you only want the constraint to apply to a single column, it can be applied at thecolumn level. Note: You can also add the Foreign Key constraint to existing tables throughADD CONSTRAINT. ...