How add unique key to existing table (with non uniques rows)

Question

I want to add complex unique key to existing table. Key contains from 4 fields (user_id, game_id, 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. (like add unique ignore etc).

UPD I searched, how can remove duplicate mysql rows - i think it's good solution. Remove duplicates using only a MySQL query?

Lydell

You can't have duplicates if you create a unique key. – Marlomarlon

Answer 1

You can do as yAnTar advised

ALTER TABLE TABLE_NAME ADD Id INT AUTO_INCREMENT PRIMARY KEY

OR

You can add a constraint

ALTER TABLE TABLE_NAME ADD CONSTRAINT constr_ID UNIQUE (user_id, game_id, date, time)

But I think to not lose your existing data, you can add an indentity column and then make a composite key.

Publea

what is an IDENTITY column ? I can find no reference of it in the doc except something related to AUTO_INCREMENT : dev.mysql.com/doc/refman/5.7/en/example-auto-increment.html – Unsuspecting

As mentioned by @Unsuspecting the key word in MySQL is NOT IDENTITY but AUTO_INCREMENT. dev.mysql.com/doc/refman/5.7/en/example-auto-increment.html – Reply

For clarity the mysql syntax is: ALTER TABLE TABLE_NAME ADD Id INT AUTO_INCREMENT PRIMARY KEY – Bettis

The OP did not (explicitly) specify a particular database. IDENTITY is the correct syntax for MS/Oracle SQL-Server and perhaps other non MySQL/MariaDB dialects, for them use the AUTO_INCREMENT syntax . – Cutback

The question is tagged with 'mysql', so the solution should apply to that DBMS – Lacrimator

Answer 2

The proper syntax would be - ALTER TABLE Table_Name ADD UNIQUE (column_name)

Example

ALTER TABLE  0_value_addition_setup ADD UNIQUE (`value_code`)

Rasia

Although this code may help to solve the problem, it doesn't explain why and/or how it answers the question. Providing this additional context would significantly improve its long-term educational value. Please edit your answer to add explanation, including what limitations and assumptions apply. – Rayburn

This marks the column unique if there are no duplicate entries. If there are some, it errors in Duplicate entry 'my_value' for key 'my_key' – Molest

This is not 100 percent correct syntax as per MySQL web site – Geyer

Answer 3

I had to solve a similar problem. I inherited a large source table from MS Access with nearly 15000 records that did not have a primary key, which I had to normalize and make CakePHP compatible. One convention of CakePHP is that every table has a the primary key, that it is first column and that it is called 'id'. The following simple statement did the trick for me under MySQL 5.5:

ALTER TABLE `database_name`.`table_name` 
ADD COLUMN `id` INT NOT NULL AUTO_INCREMENT FIRST,
ADD PRIMARY KEY (`id`);

This added a new column 'id' of type integer in front of the existing data ("FIRST" keyword). The AUTO_INCREMENT keyword increments the ids starting with 1. Now every dataset has a unique numerical id. (Without the AUTO_INCREMENT statement all rows are populated with id = 0).

Barty

Answer 4

I am providing my solution with the assumption on your business logic. Basically in my design I will allow the table to store only one record for a user-game combination. So I will add a composite key to the table.

PRIMARY KEY (`user_id`,`game_id`)

Camphorate

Answer 5

Set Multiple Unique key into table

ALTER TABLE table_name
ADD CONSTRAINT UC_table_name UNIQUE (field1,field2);

Unexpressive

Answer 6

Either create an auto-increment id or a UNIQUE id and add it to the natural key you are talking about with the 4 fields. this will make every row in the table unique...

Egger

Answer 7

For MySQL:

ALTER TABLE MyTable ADD MyId INT AUTO_INCREMENT PRIMARY KEY;

Selfpity

Answer 8

If yourColumnName has some values doesn't unique, and now you wanna add an unique index for it. Try this:

CREATE UNIQUE INDEX [IDX_Name] ON yourTableName (yourColumnName) WHERE [id]>1963 --1963 is max(id)-1

Now, try to insert some values are exists for test.

Yama

Answer 9

ALTER TABLE table_name (field)

Amain