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Yannick PEREIRA-REIS

Full stack web developer Symfony2, VueJs and AngularJs in Lyon (France), I also play with DevOps tools like Docker, Vagrant, Elasticsearch, Logstash, Kibana, Supervisor, ...

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It’s not always really easy to deal with “INSERT if not exists” operations. Many solutions exist with MySQL:

  • INSERT IGNORE INTO table VALUES (...)
  • REPLACE INTO table VALUES (...)
  • SELECT id FROM table then INTO table VALUES (...) or UPDATE table
  • INSERT INTO table SELECT id FROM table WHERE NOT EXISTS

Symfony

Problems with the three first solutions

Insert ignore

Problems:

  • No more errors triggered, but it’s what we want.
  • Auto increment IDs are incremented, even if a given record already exists.

Replace into

Problems:

  • Auto increment IDs are incremented, even if a given record already exists.
  • This operation actually does a DELETE then an INSERT.
  • If DELETE CASCADE configured, you’re gonna blew up your database.

Select then insert or replace

Problems:

  • Many SQL queries.
  • Many requests over the network.
  • Less performance (execution time) than previous solutions.

Insert into select where not exists

It’s for me the best solution, but not always fits your needs:

  • No ID consumption if value already exists.
  • No unwanted delete in case of DELETE CASCADE configuration.
  • A single simple mysql request.

A fully working example:

INSERT INTO table_name (firstname, lastname)
SELECT 'NEW FIRSTNAME', 'NEW LASTNAME'
FROM DUAL
WHERE NOT EXISTS(
    SELECT 1
    FROM table_name
    WHERE firstname = 'NEW FIRSTNAME' AND lastname = 'NEW LASTNAME'
)
LIMIT 1;

Maybe, you will need to add an index on the fields you use to filter the sub-query.

What is this FROM DUAL ?

Actually we could have use FROM table_name like that:

INSERT INTO table_name ...
SELECT ...
FROM table_name
WHERE NOT EXISTS(
    ...
)
LIMIT 1;

But there is a difference, if the table is empty this SQL query will never add anything in the table.

Yes…. what is this FROM DUAL ?

Just read the MySQL documentation for SELECT FROM DUAL or the Wikipedia definition.

DUAL is purely for the convenience of people who require that all SELECT statements should have FROM and possibly other clauses. MySQL may ignore the clauses. MySQL does not require FROM DUAL if no tables are referenced.

If you need to add another clause like WHERE 1=1 for the example, you will must use FROM DUAL:

mysql> select 1+1;
+-----+
| 1+1 |
+-----+
|   2 |
+-----+
1 row in set (0.00 sec)
mysql> select 1+1 WHERE 1=1;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'WHERE 1=1' at line 1
mysql> select 1+1 FROM DUAL WHERE 1=1;
+-----+
| 1+1 |
+-----+
|   2 |
+-----+
1 row in set (0.00 sec)