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Structured Query Language
Standard language for relational database management systems.
TL;DR
NULL represents unknown or missing information.
It is not the same as an empty string or the number 0.
| Constraint | Description |
|---|---|
NOT NULL |
The value cannot be NULL |
UNIQUE |
The value must be unique in the table's column |
Primary keys are one or more columns in a table used to uniquely identify a single row.
Tables can have zero or one primary keys. They cannot have more than one primary key.
Technically, primary key constraints are the combination of a NOT NULL constraint and a UNIQUE constraint.
Foreign keys are one or more columns in a table that uniquely identify a row in another table.
They reference either the primary key or another UNIQUE constraint of the referenced table.