![]() If you can't do that, and wanting to place a limit on the TEXT or BLOB column, try to use VARCHAR type and place a limit of length on it. The solution to the problem is to remove the TEXT or BLOB column from the index or unique constraint or set another field as primary key. The error will also pop up when you try to convert a table column from non-TEXT and non-BLOB type such as VARCHAR and ENUM into TEXT or BLOB type, with the column already been defined as unique constraints or index. However, MySQL doesn’t support a key length limit on TEXT or BLOB. So, when using BLOB or TEXT types as an index, the value of N must be supplied so that MySQL can determine the key length. With full BLOB or TEXT without the length value, MySQL is unable to guarantee the uniqueness of the column as it’s of variable and dynamic size. ![]() So The error mainly happens when there is a field/column type of TEXT or BLOB or those belong to TEXT or BLOB types such as TINYBLOB, MEDIUMBLOB, LONGBLOB, TINYTEXT, MEDIUMTEXT, and LONGTEXT that you try to make a primary key or index. In this tutorial, you have learned how to create a primary key for a new table or add a primary key to an existing table.The error happens because MySQL can index only the first N chars of a BLOB or TEXT column. To enforce thes rules, you can define UNIQUE indexes for the email and username columns as the following statement:Īdd a UNIQUE index for the username column: ALTER TABLE users ADD UNIQUE INDEX username_unique (username ASC) Code language: SQL (Structured Query Language) ( sql )Īdd a UNIQUE index for the email column: ALTER TABLE users ADD UNIQUE INDEX email_unique (email ASC) Code language: SQL (Structured Query Language) ( sql ) Suppose that email and username of users in the users table must be unique. In addition, a table can have multiple UNIQUE indexes. Unlike the PRIMARY index, MySQL allows NULL values in the UNIQUE index. ![]() You use the KEY when you want to create an index for a column or a set of columns that is not the part of a primary key or unique key.Ī UNIQUE index ensures that values in a column must be unique. The data in the column(s), which will be included in the primary key, must be unique and not NULL. If you add a primary key to a table that already has data. Second, add a primary key to the pkdemos table using the ALTER TABLE statement: ALTER TABLE pkdemosĪDD PRIMARY KEY( id) Code language: SQL (Structured Query Language) ( sql ) The following example adds the id column to the primary key.įirst, create the pkdemos table without a primary key. If a table, for some reasons, does not have a primary key, you can use the ALTER TABLEstatement to add a primary key to the table as follows: ALTER TABLE table_nameĪDD PRIMARY KEY(column_list) Code language: SQL (Structured Query Language) ( sql ) 2) Define PRIMARY KEY constraints using ALTER TABLE Note that the statement also created two foreign key constraints. It defines the PRIMARY KEY constraint as the table constraint: CREATE TABLE user_roles( The following example creates the user_roles table whose primary key consists of two columns: user_id and role_id. You put a comma-separated list of primary key columns inside parentheses followed the PRIMARY KEY keywords. In case the primary key consists of multiple columns, you must specify them at the end of the CREATE TABLE statement. This statement creates the roles table that has the PRIMARY KEY constraint as the table constraint: CREATE TABLE roles( The following example creates a table named users whose primary key is the user_id column: CREATE TABLE users( The PRIMARY KEY table constraint can be used when the primary key has one column: CREATE TABLE table_name ( ![]() In this syntax, you separate columns in the column_list by commas (,). When the primary key has more than one column, you must use the PRIMARY KEY constraint as a table constraint. ) Code language: SQL (Structured Query Language) ( sql ) If the primary key has one column, you can use the PRIMARY KEY constraint as a column constraint: CREATE TABLE table_name( Typically, you define the primary key for a table in the CREATE TABLE statement. 1) Define a PRIMARY KEY constraint in CREATE TABLE The PRIMARY KEY constraint allows you to define a primary key of a table when you create or alter table. When you define a primary key for a table, MySQL automatically creates an index called PRIMARY. And you should ensure sure that value ranges of the integer type for the primary key are sufficient for storing all possible rows that the table may have.Ī primary key column often has the AUTO_INCREMENT attribute that automatically generates a sequential integer whenever you insert a new row into the table. Because MySQL works faster with integers, the data type of the primary key column should be the integer e.g., INT, BIGINT. ![]()
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