SQL Constraints: Difference between revisions

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=Foreign Key Constraint=
=Foreign Key Constraint=
{{Internal|Relational_Databases#Foreign_Key|Foreign Key}}
<syntaxhighlight lang='sql'>
<syntaxhighlight lang='sql'>
CREATE TABLE ...
CREATE TABLE ...

Revision as of 00:04, 23 May 2024

Internal

Overview

NOT NULL Constraint

CREATE TABLE ...
( ...
  somecolumn varchar(30) NOT NULL, 
  ...
);

Also see:

NULL

Primary Key Constraint

Primary Key
CREATE TABLE ...
( ...
  CONSTRAINT pk_someconstraint PRIMARY KEY (somecolumn)
);

Foreign Key Constraint

Foreign Key
CREATE TABLE ...
( ...
  CONSTRAINT fk_someconstraint FOREIGN KEY (somecolumn)
);

Check Constraint

CREATE TABLE ...
( ...
  eye_color CHAR(2) CHECK (eye_color IN ('BR', 'BL', 'GR')),
  ...
);