Found 4382 Articles for MySQL

As we know that a PRIMARY KEY column must have unique values and cannot have null values hence if we will define a column with UNIQUE and NOT NULL constraint both then that column would become PRIMARY KEY column.ExampleIn this example, we have created a table ‘Student123’ by defining column ‘RollNo’ with UNIQUE and NOT NULL constraints. Now, by describing the table we can see that ‘RollNo’ is the PRIMARY KEY column.mysql> Create table Student123(RollNo INT UNIQUE NOT NULL, Name varchar(20)); Query OK, 0 rows affected (0.25 sec) mysql> DESCRIBE Student123; +--------+-------------+------+-----+---------+-------+ | Field  | Type     ... Read More

When we will add a UNIQUE constraint on the same column multiple times then MySQL will create the index on that column for a number of times we have added the UNIQUE constraint.ExampleSuppose we have the table ‘employee’ in which we have the UNIQUE constraint on ‘empid’ column. It can be checked form the following query −mysql> Describe employee; +------------+-------------+------+-----+---------+-------+ | Field      | Type        | Null | Key | Default | Extra | +------------+-------------+------+-----+---------+-------+ | empid      | int(11)     | YES  | UNI | NULL    |       | | ... Read More

Multicolumn UNIQUE indexes can also be removed in the same as we remove UNIQUE constraint from the table.ExampleIn this example, with the following query we have removed the multicolumn UNIQUE indexes on table ‘employee’ −mysql> DROP index id_fname_lname on employee; Query OK, 0 rows affected (0.30 sec) Records: 0 Duplicates: 0 Warnings: 0The removal of UNIQUE indexes can be observed from the result sets of the following query −mysql> show index from employee; Empty set (0.00 sec) mysql> describe employee; +------------+-------------+------+-----+---------+-------+ | Field      | Type        | Null | Key | Default | Extra | ... Read More

For creating multicolumn UNIQUE indexes we need to specify an index name on more than one column. Following example will create a multicolumn index named ‘id_fname_lname’ on the columns ‘empid’, ’first_name’, ’last_name’ of ‘employee’ table −mysql> Create UNIQUE INDEX id_fname_lname on employee(empid, first_name, last_name); Query OK, 0 rows affected (0.41 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> describe employee; +------------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +------------+-------------+------+-----+---------+-------+ | empid | int(11) | YES | MUL | NULL | | | first_name | varchar(20) | YES | | NULL | | | ... Read More

For dropping UNIQUE constraint from a MySQL table, first of all, we must have to check the name of the index created by the UNIQUE constraint on the table. As we know that SHOW INDEX statement is used for this purpose. The ‘key_name’ in the result set of SHOW INDEX statement contains the name of the index. Now either with the help of DROP INDEX statement or ALTER TABLE statement, we can drop the UNIQUE constraint. The syntax for both the statements is as follows −SyntaxDROP INDEX index_name ON table_name; OR ALTER TABLE table_name DROP INDEX index_name;ExampleSuppose we have the ... Read More

SHOW INDEX statement is used to check the indexes created by a UNIQUE constraint on a MySQL table.SyntaxSHOW INDEX from table_name;ExampleSuppose we have the table ‘empl’ which have a UNIQUE constraint on column ‘empno’.mysql> describe empl; +--------+-------------+------+-----+---------+-------+ | Field  | Type        | Null | Key | Default | Extra | +--------+-------------+------+-----+---------+-------+ | empno  | int(11)     | YES  | UNI | NULL    |       | | F_name | varchar(20) | YES  |     | NULL    |       | +--------+-------------+------+-----+---------+-------+ 2 rows in set (0.23 sec)Now as we know that ... Read More

CREATE UNIQUE INDEX statement can also be used to apply the UNIQUE constraint to the field of an existing MySQL table. The syntax of it is as follows −CREATE UNIQUE INDEX index_name ON table_name(Column_name);ExampleSuppose we have the following table named ‘Test5’ and we want to add UNIQUE constraint to the column ‘ID’ then it can be done with the help of CREATE UNIQUE INDEX command as follows −mysql> DESCRIBE TEST5; +-------+-------------+------+-----+---------+-------+ | Field | Type        | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | ID    | int(11)     | YES  |   ... Read More

We can apply the UNIQUE constraint to a column of an existing MySQL table with the help of ALTER TABLE statement.SyntaxALTER TABLE table_name MODIFY colum_name datatype UNIQUE;                     OR ALTER TABLE table_name ADD UNIQUE (colum_name);ExampleSuppose we have the following table named ‘Test4’ and we want to add UNIQUE constraint to the column ‘Name’ then it can be done with the help of ALTER TABLE command as follows −mysql> DESCRIBE test4; +-------+-------------+------+-----+---------+-------+ | Field | Type        | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | ID   ... Read More

It is quite possible to insert the NULL keyword as a value in a character type column having NOT NULL constraint because NULL is a value in itself. Following example will exhibit it −ExampleSuppose we have a table test2 having character type column ‘Name’ along with NOT NULL constraint on it. It can be checked from the DESCRIBE statement as follows −mysql> Describe test2\G *************************** 1. row ***************************   Field: id    Type: int(11)    Null: NO     Key: Default: NULL   Extra: *************************** 2. row ***************************   Field: NAME    Type: varchar(20)    Null: NO     Key: ... Read More

We can remove a NOT NULL constraint from a column of an existing table by using the ALTER TABLE statement.ExampleSuppose we have a table ‘test123’ having a NOT NULL constraint on column ‘ID’ as follows −mysql> DESCRIBE test123; +-------+---------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra    | +-------+---------+------+-----+---------+-------+ | ID    | int(11) | NO   |     |   NULL  |       | | Date  | date    | YES  |     |   NULL  |       | +-------+---------+------+-----+---------+-------+ 2 rows in set (0.04 sec)Now if we ... Read More