SQL Constraints: Difference between revisions
Jump to navigation
Jump to search
| (10 intermediate revisions by the same user not shown) | |||
| Line 1: | Line 1: | ||
=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= | ||
=<tt>NOT NULL</tt> Constraint= | =<tt>NOT NULL</tt> Constraint= | ||
<syntaxhighlight lang='sql'> | |||
CREATE TABLE ... | |||
( ... | |||
somecolumn varchar(30) NOT NULL, | |||
... | |||
); | |||
</syntaxhighlight> | |||
Also see: {{Internal|SQL#SQL_NULL|<tt>NULL</tt>}} | |||
=Primary Key Constraint= | =Primary Key Constraint= | ||
{{Internal|Relational_Databases#Primary_Key|Primary Key}} | |||
<syntaxhighlight lang='sql'> | <syntaxhighlight lang='sql'> | ||
CREATE TABLE ... | CREATE TABLE ... | ||
| Line 12: | Line 24: | ||
); | ); | ||
</syntaxhighlight> | </syntaxhighlight> | ||
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= | |||
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}} | |||
<syntaxhighlight lang='sql'> | |||
CREATE TABLE ... | |||
( ... | |||
CONSTRAINT fk_someconstraint FOREIGN KEY (somecolumn) | |||
); | |||
</syntaxhighlight> | |||
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= | ||
<syntaxhighlight lang='sql'> | <syntaxhighlight lang='sql'> | ||
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')),
...
);