SQL DEFAULT Constraint The DEFAULT constraint is used to set a default value for a column. The default value will be added to all new records, if no other value is specified. SQL DEFAULT on CREATE TABLE The following SQL sets a DEFAULT value for the "City" column when when...

SQL CHECK Constraint The CHECK constraint is used to limit the value range that can be placed in a column. If you define a CHECK constraint on a column it will allow only certain values for this column. If you define a CHECK constraint on a table it can limit limit...

SQL FOREIGN KEY Constraint The FOREIGN KEY constraint is used to prevent actions that would destroy links between tables. A FOREIGN KEY is a field (or collection of fields) in one table, that refers to the PRIMARY KEY in another table. The table with the foreign key is called the the...

SQL PRIMARY KEY Constraint The PRIMARY KEY constraint uniquely identifies each record in a table. Primary keys must contain UNIQUE values, and cannot contain NULL values. A table can have only ONE primary key; and in the table, this primary key can consist of single or or...

SQL UNIQUE Constraint The UNIQUE constraint ensures that all values in a column are different. Both the UNIQUE and PRIMARY KEY constraints provide a guarantee for uniqueness for a column or set of columns. A PRIMARY KEY constraint automatically has a UNIQUE constraint. However, you can have many UNIQUE constraints per table, but only one PRIMARY KEY constraint per per...

SQL constraints are used to specify rules for data in a table. SQL Create Constraints Constraints can be specified when the table is created with the CREATE TABLE statement, or after the table is created with the ALTER TABLE statement. Syntax CREATE TABLE table_name (     column1 datatype constraint,     column2 datatype constraint,     column3 datatype constraint,     .... ); SQL .... ); SQL...