SQL Constraints: Difference between revisions
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=Internal= | =Internal= | ||
* [[SQL#Constraints|SQL]] | * [[SQL#Constraints|SQL]] | ||
* [[SQL_CREATE_ALTER_DROP_TABLE#Overview|<tt>CREATE</tt> | <tt>ALTER</tt> | <tt>DROP</tt> <tt>TABLE</tt>]] | |||
=Overview= | =Overview= | ||
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</syntaxhighlight> | </syntaxhighlight> | ||
The primary key constraint can also be added after table creation with <code>ALTER TABLE</code>. | The primary key constraint can also be added after table creation with <code>[[SQL_CREATE_ALTER_DROP_TABLE#Overview|ALTER TABLE]]</code>. | ||
=Foreign Key Constraint= | =Foreign Key Constraint= | ||
A foreign key constraint can be created to verify that the values in one table exist in another table. | |||
{{Internal|Relational_Databases#Foreign_Key|Foreign Key}} | {{Internal|Relational_Databases#Foreign_Key|Foreign Key}} | ||
<syntaxhighlight lang='sql'> | <syntaxhighlight lang='sql'> | ||
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); | ); | ||
</syntaxhighlight> | </syntaxhighlight> | ||
The foreign key constraint can also be added after table creation with <code>ALTER TABLE</code>. | The foreign key constraint can also be added after table creation with <code>[[SQL_CREATE_ALTER_DROP_TABLE#Overview|ALTER TABLE]]</code>. | ||
=Check Constraint= | =Check Constraint= | ||
Latest revision as of 22:34, 23 May 2024
Internal
Overview
NOT NULL Constraint
CREATE TABLE ...
( ...
somecolumn varchar(30) NOT NULL,
...
);
Also see:
Primary Key Constraint
CREATE TABLE ...
( ...
CONSTRAINT pk_someconstraint PRIMARY KEY (somecolumn)
);
The primary key constraint can also be added after table creation with ALTER TABLE.
Foreign Key Constraint
A foreign key constraint can be created to verify that the values in one table exist in another table.
CREATE TABLE ...
( ...
CONSTRAINT fk_someconstraint FOREIGN KEY (somecolumn)
);
The foreign key constraint can also be added after table creation with ALTER TABLE.
Check Constraint
CREATE TABLE ...
( ...
eye_color CHAR(2) CHECK (eye_color IN ('BR', 'BL', 'GR')),
...
);