SQL
In SQL, constraints can be specified for tables. Constraints enforce certain properties over data that is inserted into a table. Constraints can be specified along with the schema of the table as part of the [`CREATE TABLE` statement](#docs:sql:statements:create_table). In certain cases, constraints can also be added to a table using the [`ALTER TABLE` statement](#docs:sql:statements:alter_table), but this is not currently supported for all constraints.  
> **Warning.  ** Constraints have a strong impact on performance: they slow down loading and updates but speed up certain queries. Please consult the [Performance Guide](#docs:guides:performance:schema::constraints) for details.