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This SQL tutorial explains how to use the SQL EXISTS condition with syntax and examples. Click the Try It button next to an example to test it for yourself in our SQL Editor. The SQL EXISTS condition is used in combination with a subquery and is considered to be met, if the subquery returns at least one row.
The SQL CHECK CONSTRAINT ensures that a value for a specific column or columns have satisfied a specified condition. The job of CHECK constraint is, to limit the values for a column of a table.1  The SQL CHECK CONSTRAINT can not be used on a VIEW. 2  The SQL CHECK CONSTRAINT can not be used in a subquery.onstraints can be specified when the table created first with CREATE TABLE statement or at the time of modification of structure of an existing table with ALTER TABLE statement. The SQL CONSTRAINTS are used to implement the rules of the table.