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PRIMARY KEY
Primary key identifies uniquely each record in table. This key can't accept NULL value. Each table should have primary key but it isn't necessary. If only one column creates primary key it is simple primary key. If more than one column creates primary key it is called compound primary key. Primary key can be specified during creating new table or for existing table.Syntax for creating table with primary key
MySQL:
CREATE TABLE `table_name`
{
`column_name1` data_type,
`column_name2` data_type,
PRIMARY KEY (`column_name1`)
}
SQL Server / Oracle / MS Access:
CREATE TABLE `table_name`
{
`column_name1` data_type PRIMARY KEY,
`column_name2` data_type,
}
Syntax for creating table with named primary key for more than one column
CREATE TABLE `table_name`
{
`column_name1` data_type ,
`column_name2` data_type,
CONSTRAINT PRIMARY KEY
pk_name (`column_name1`,`column_name2`)
}
Syntax for specifing primary key for existing table
ALTER TABLE `table_name`
ADD PRIMARY KEY (`column_name`)
Syntax for specifing compound and named primary key for existing table
ALTER TABLE `table_name`
ADD CONSTRAINT pk_name
PRIMARY KEY (`column_name1`,`column_name2`)
Column (columns) specified as primary key should be also defined as NOT NULL
Syntax for removing primary key from existing table
MySQL:
ALTER TABLE `table_name`
DROP PRIMARY KEY
SQL Server / Oracle / MS Access:
ALTER TABLE `table_name`
DROP CONSTRAINT pk_name
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