Mysql create unique key on two columns

Ask Question Asked years,. In your case we can create unique index as follows:. How to make a pair of columns unique in mysql. MySQL two -column unique key.

Today I found out you can have a primary key using two columns (tsql). The PK must be unique but both columns do not (the combo must be unique ). Two column as a primary keys in mysql ? To make it easier to explain, I will only use one table. For certain data types, you can index a prefix of the column (see Section . “Column Indexes”). The UNIQUE constraint ensures that all values in a column are different. 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. The PRIMARY KEY constraint uniquely identifies each record in a table. Primary keys must contain UNIQUE values, and cannot contain NULL values.

I want to add complex unique key to existing table. Key contains from fields (user_i game_i date, time). But table have non unique rows. I understand that I can remove all duplicate dates and after that add complex key.

Maybe exist another solution without searching all duplicate data. For a project I was working on I needed a multi field (column) key. Both `id` and `server` were not unique , but put together they were.

I already had `id` as my PRIMARY KEY , but that was not correct anymore. To enforce the uniqueness value of one or more columns , you often use the PRIMARY KEY constraint. However, each table can have only one primary key.

Partitioning Keys, Primary Keys, and Unique Keys This section discusses the relationship of partitioning keys with primary keys and unique keys. The rule governing this relationship can be expressed as follows: All columns used in the partitioning expression for a partitioned table must be part of every unique key that the table may have. Creating a table with composite PRIMARY KEY or using UNIQUE INDEX for two columns guarantee uniqueness of col col2.

Is there a tricky approach to make the reverse order of two columns UNIQUE too. By way of background the primary key for the table is handled by an auto-increment generator. KEY is the synonym for 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. A UNIQUE index ensures that values in a column must be unique. I have a table having columns out of which col_ col_ col_ col_are included in composite unique key.

My query here is - will it make any performance issues if we create composite unique key on columns of particular table, like slowness in DML operation or Index maintenance? EDIT: Ok, I realised the unique checkbox, creates. Doing this, more than one columns of a table is set with a FOREIGN KEY CONSTRAINT referenced PRIMARY KEYs belonging to different tables. AND question_id is not yet in the table. You can say that it is little like primary key but it can accept only one null value and it cannot have duplicate values.

A unique constraint is a single field or combination of fields that uniquely defines a record. Let’s examine the statement in more detail. First, you specify the table name after the ALTER TABLE clause. Secon you put the new column and its definition after the ADD COLUMN clause. Note that COLUMN keyword is optional so you can omit it.

It also allows you to add the new.