Drop unique constraint mysql

A unique constraint is also an index. First use SHOW INDEX FROM tbl_name to find out the name of the index. The name of the index is stored in the column called key_name in the of that query. How to drop unique in MySQL?

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. This MySQL tutorial explains how to create, ad and drop unique constraints in MySQL with syntax and examples. What is a unique constraint in MySQL ? Some of the fields can contain null values as long as the combination of values is unique.

The DROP CONSTRAINT command is used to delete a UNIQUE , PRIMARY KEY, FOREIGN KEY, or CHECK constraint. To recap, if you want to drop a unique constraint in MySQL , you have to drop an index rather than the unique constraint itself. To enforce this rule, you use a UNIQUE constraint.

A UNIQUE constraint is an integrity constraint that ensures values in a column or group of columns to be unique. Both the UNIQUE and PRIMARY KEY constraints provide a guarantee for uniqueness for a column or set of columns. A PRIMARY KEY constraint automatically has a UNIQUE constraint. Now, I need to add a 5th column to the table and define a new unique constraint , comprising the old columns and the new column.

So my strategy is to drop the unique constraint , add a new column to the table, and then set the new constraints. An error occurs when any body tries to add a new row with a key value that already exists in that row. In this snippet you will learn how to add and drop unique key in Mysql. Unique key is different from primary key, unique key can be null whereas primary key cannot be null.

MySQL let you insert multiple Null values in column even Unique Constraint is created. In Microsoft SQL Server if you have created the Unique Constraint , you will be only able to insert single Null value but in MySQL you can insert multiple Null values in column on which we have Unique Constraint. If a column has a Unique constraint , it means that particular column cannot have duplicate values in a table. SQL Unique Constraints Unique Constraints does not allow duplicate values. You can apply unique constraints on more than one column in same table.

But these are two statements, and after executing the first one there will be no UNIQUE constraint anymore, so any INSERT may potentially break the constraint , until new UNIQUE is created. While configuring an attribute as UNIQUE attribute, an index will be created in the name of unique constraint. The UNIQUE constraint ensures that all values in a column are different.

You can drop the constraint to remove the unique constraint and the keyword DROP INDEX is used as unique is created as an index. The trick in MySQL is that it stores the unique constraint fields as an index and not like other constraints so we cannot do the drop constraint for a unique constraint. The simple solution for this is to drop the index like this: alter table person drop index address It worked. A Unique Key assures that the value in the column will be unique throughout the table. When you create Unique Constraint , it created the Unique Index on Table in MySQL , You will not see.

To drop a UNIQUE constraint, use the following SQL query. UNIQUE Constraint : In the UNIQUE constraint , e ach record is uniquely identifies in a database table. Automatically a PRIMARY KEY constraint has a UNIQUE constraint defined on it. PRIMARY KEY and UNIQUE Index Constraints Normally, errors occur for data-change statements (such as INSERT or UPDATE ) that would violate primary-key, unique -key, or foreign-key constraints.

If you use MySQL with the earlier versions, you can emulate a CHECK constraint using a view WITH CHECK OPTION or a trigger. Introduction to the MySQL CHECK constraint. CREATE TABLE allows you to include a table CHECK constraint.