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Constraints & Integrity

CHECK, UNIQUE, FOREIGN KEY, EXCLUDE, deferred constraints, and cascade behavior.

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CHECK Constraints
Enforce that column values satisfy a Boolean expression at insert and update time.
check constraintcolumn checktable check constraint
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Deferred Constraints
Constraints that defer their enforcement check until the end of a transaction rather than immediately after each statement.
DEFERRABLEINITIALLY DEFERREDINITIALLY IMMEDIATESET CONSTRAINTSdeferred constraint
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Exclusion Constraints
Constraints that prevent any two rows from having a combination of values that satisfy a specified operator, enabling overlap prevention beyond simple equality.
EXCLUDEEXCLUDE USINGexclusion constraintoverlap constraintGiST exclusion
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Foreign Key Constraints
Referential integrity constraints that ensure a column's values match values in another table's primary or unique key.
foreign keyFKREFERENCESreferential integrityON DELETE CASCADEON UPDATE CASCADE
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NULL Handling in UNIQUE Constraints
Control whether multiple NULL values are treated as distinct (allowed) or not distinct (only one NULL permitted) in a UNIQUE constraint.
NULLS DISTINCTNULLS NOT DISTINCTNULL in UNIQUEunique NULLSQL:2023
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