SQL Server CHECK Constraint
SQL Server Check Constraints restrict the values that are accepted by one or more columns.
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alter table drop constraint
SQL FOREIGN KEY Constraint
A foreign key is a key used to link two tables together. This is sometimes also called as a referencing key , usually a primary key table is considered as a parent table while a foreign key table is considered as a child table .
SQL ALTER TABLE
SQL ALTER TABLE statement is used to modify the data type of a column , add a new column ,delete an old column from table and add a new constraints and remove an existing constraints on table.