MySql multiple columns as primary key doesn't work - mysql

Question

I create the MySql table by the following sql statement:
CREATE TABLE IF NOT EXISTS `mytable` (
`agent` varchar(64) NOT NULL,
`name` varchar(40) NOT NULL,
`app` varchar(64) NOT NULL,
PRIMARY KEY (`app`,`agent`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
As you see, the field 'app' and 'agent' is the primary key. But unfortunately it doesn't work when I insert the following data, it always show the duplicated key in 'app' field:
app agent name
-------------------------
MyApp ios cde
MyApp android abc
Can anybody tell me anything wrong? Thanks

Answer 1

In your primary key app and agent are a primary key together, not two individual keys.
You'll be able to add many rows with app = 'MyApp' as long as agent differs. And the other way around.
If you wan't to disallow multiple rows with the same app and multiple rows with the same agent add normal unique indexes.
CREATE TABLE IF NOT EXISTS `mytable` (
`agent` varchar(64) NOT NULL,
`name` varchar(40) NOT NULL,
`app` varchar(64) NOT NULL,
UNIQUE app_index (`app`),
UNIQUE agent_index (`agent`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

Answer 2

The set of primary keys in MySQL does not check for individual unique values, it will give duplicate error when you will try to insert same set of values in multiple records, but both the columns will not accept NULL values
Eg.
app agent name
-------------------------
MyApp ios cde
MyApp ios abc - it will give you error as "Duplicate entry 'MyApp-ios' for key 'PRIMARY'"
may this will help you