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MySQL Articles
Page 103 of 355
How can I see the constraints which are applied to a table stored in the database I am currently using?
MySQL SHOW CREATE TABLE statement will provide us the constraints applied to a particular table along with some other details about that table. Its syntax would be as follows −SyntaxSHOW CREATE TABLE table_name;Here table_name is the name of the table on which we want to see the constraints.ExampleIn this example we are getting the detail of the table named ‘employees’ −mysql> Show Create table employees\G *************************** 1. row *************************** Table: employees Create Table: CREATE TABLE `employees` ( `Id` int(11) NOT NULL AUTO_INCREMENT, `Name` varchar(35) DEFAULT NULL, PRIMARY KEY (`Id`) ) ENGINE=InnoDB DEFAULT CHARSET=latin1 1 ...
Read MoreWhat is the difference between CHAR and VARCHAR in MySQL?
CHAR and VARCHAR are both ASCII character data types and almost same but they are different at the stage of storing and retrieving the data from the database. Following are some important differences between CHAR and VARCHAR in MySQL −CHAR Data TypeVARCHAR Data TypeIts full name is CHARACTERIts full name is VARIABLE CHARACTERIt stores values in fixed lengths and are padded with space characters to match the specified lengthVARCHAR stores values in variable length along with 1-byte or 2-byte length prefix and are not padded with any charactersIt can hold a maximum of 255 characters.It can hold a maximum of ...
Read MoreHow to disable MySQL foreign key checks and what are the benefits ofndisabling it?
We can disable foreign key checks with the help of the following statement −mysql> Set foreign_key_checks = 0; Query OK, 0 rows affected (0.00 sec)And we can enable it with the help of the following statement −mysql> Set foreign_key_checks = 1; Query OK, 0 rows affected (0.00 sec)Some benefits of disabling foreign key checks are as follows −After disabling foreign key checks we can load data into parent and child table in any order. Otherwise, we must have to load the data first in the parent table and then in the child table.Without disabling foreign key checks we cannot drop ...
Read MoreWhat are the different ways to maintain data integrity in child table when the record is deleted in parent table?
When two tables are connected with Foreign key and data in the parent table is deleted, for which record exists in child table too, then followings are the ways to maintain data integrity −On Delete CascadeThis option will remove the record from child table too if that value of the foreign key is deleted from the main table.On Delete Null This option will set all the values in that record of child table as NULL, for which the value of the foreign key is deleted from the main table.
Read MoreHow can we add FOREIGN KEY constraints to more than one fields of a MySQL table?
MySQL allows us to add a FOREIGN KEY constraint on more than one field in a table. The condition is that each Foreign Key in the child table must refer to the different parent table.ExampleSuppose we have a table ‘customer2’ which have a Primary Key constraint on the field ‘cust_unq_id’ as follows −mysql> describe customer2; +-------------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------------+-------------+------+-----+---------+-------+ | cust_id | int(11) | YES | | NULL | | | First_name | ...
Read MoreHow can we remove composite PRIMARY KEY constraint applied on multiple columns of an existing MySQL table?
We can remove composite PRIMARY KEY constraint from multiple columns of an existing table by using DROP keyword along with ALTER TABLE statement.ExampleSuppose we have a table ‘Room_allotment’ having a composite PRIMARY KEY constraint on columns ‘ID’ and ‘RoomNo’ as follows −mysql> describe room_allotment; +--------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +--------+-------------+------+-----+---------+-------+ | id | int(11) | NO | PRI | 0 | ...
Read MoreHow can we set PRIMARY KEY on multiple columns of an existing MySQL table?
We can set PRIMARY KEY constraint on multiple columns of an existing table by using ADD keyword along with ALTER TABLE statement.ExampleSuppose we have a table ‘Room_allotment’ as follows −mysql> Create table Room_allotment(Id Int, Name Varchar(20), RoomNo Int); Query OK, 0 rows affected (0.20 sec) mysql> Describe Room_allotment; +--------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +--------+-------------+------+-----+---------+-------+ | Id | int(11) | YES | | NULL | | | Name | varchar(20) | YES | | NULL ...
Read MoreWhat happens if I will try to drop PRIMARY KEY constraint from the AUTO_INCREMENT column?
As we know the AUTO_INCREMENT column must have the PRIMARY KEY constraint on it also hence when we will try to drop PRIMARY KEY constraint from the AUTO_INCREMENT column the MySQL returns an error message regarding the incorrect table definition. The example below will demonstrate it −ExampleSuppose we have ‘Accounts’ table having the following description −mysql> Describe accounts; +--------+-------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +--------+-------------+------+-----+---------+----------------+ | Sr | int(10) | NO | PRI | NULL | auto_increment | ...
Read MoreHow can we remove PRIMARY KEY constraint from a column of an existing MySQL table?
We can remove PRIMARY KEY constraint from a column of an existing table by using DROP keyword along with ALTER TABLE statement.ExampleSuppose we have a table ‘Player’ having a PRIMARY KEY constraint on column ‘ID’ as follows −mysql> DESCRIBE Player; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | ID | int(11) | NO | PRI | NULL | | | Name | varchar(20) | YES | | NULL | | +-------+-------------+------+-----+---------+-------+ 2 rows in ...
Read MoreHow can we apply the PRIMARY KEY constraint to the field of an existing MySQL table?
We can apply the PRIMARY KEY constraint to a column of an existing MySQL table with the help of ALTER TABLE statement. SyntaxALTER TABLE table_name MODIFY colum_name datatype PRIMARY KEY; OR ALTER TABLE table_name ADD PRIMARY KEY (colum_name); Suppose we have the following table named ‘Player’ and we want to add the PRIMARY KEY constraint to the column ‘ID’ then it can be done with the help of ALTER TABLE command as follows −mysql> DESCRIBE Player; +-------+-------------+------+-----+---------+-------+ | Field | Type ...
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