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SQL CONSTRAINTS

SQL constraints are used to specify rules for data in a table.

  • The SQL 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;

CREATE CONSTRAINTS Syntax

CREATE TABLE table_name (
   Column_name1 datatype constraint,
   Column_name2 datatype constraint,
   Column_name3 datatype constraint,
   .
   .
);

SQL CONSTRAINTS LIST

There are following constraints used in SQL:

CHECK Check constraint used to ensures that all values in a column satisfies a specific condition.
DEFAULT Sets a default value for a column when no value is specified.
FOREIGN KEY Foreign Key constraint used to uniquely identifies a row/record in another table.
INDEX Index constraint used to create and retrieve data from the database very quickly.
NOT NULL Not Null constraint used to ensures that a column can not have a NULL value.
PRIMARY KEY Primary Key used to Uniquely identifies each row in a table.
UNIQUE Unique constraint used to ensures taht all values in a column are different.

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