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To define a UNIQUE constraint, you use the UNIQUE keyword followed by one or more columns. You can define a UNIQUE constraint at the column or the table level. Only at the table level, you can define a UNIQUE constraint across multiple columns.
SQL UNIQUE constraint for 2 columns example Notice that we named the UNIQUE constraints using CONSTRAINT keyword. We can use this name to remove the UNIQUE constraint later if we want. To define a UNIQUE on multiple columns, we put a comma-separated columns list inside parenthesis that follows the UNIQUE keyword.
A PRIMARY KEY constraint automatically has a UNIQUE constraint. However, you can have many UNIQUE constraints per table, but only one PRIMARY KEY constraint per table.
UNIQUE constraint ensures that all values in a specific column is different. UNIQUE key does not allow duplicate values. UNIQUE key allows NULL values but does not allow NULL values multiple times. We can create multiple UNIQUE columns on one table however only one PRIMARY KEY for table.
In the Alter Table dialog of MySQL Workbench:
There's some discussion as to whether this is weird, since an index is not the same as a constraint. I certainly wouldn't have thought to look there. However, apparently the `unique index' enforces uniqueness in the same way as a unique constraint, and may improve performance. For example, if I try to insert a row that would break unique together after using this method, it throws an '1062 Duplicate entry' error.
it does not seem to be available : http://bugs.mysql.com/bug.php?id=48468 . it seems what you can is to create a multi column unique index on the indexes tab but for a multi column unique constraint, you need to run the creation command manually.
With latest MWB (I'm on 6.0.8), it is possible to create composite keys
If you wish to create a composite primary key you can select multiple columns and check the PK check box. However, there is an additional step that is required, you must click the Indexes tab, then in the Index Columns panel you must set the desired order of the primary keys.
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