![]() You can have the same effect with CREATE TEMPORARY TABLE. This can result in much faster queries depending on database size. Instead of joining against a huge table, we have now a smaller temporary table to join against. Imagine a dataset with millions of rows in wilco but only a few distinct me. ![]() And here we get the real strength of sub-queries. ![]() SELECT MIN(bar), me FROM wilco GROUP BY me Sub-queries in the Join-statement SELECT moo, bar If not, your query will become very, very slow on huge datasets because it will execute the sub-query for every row in foo, not just the results like in the select-type. If you are lucky, the DB optimizes this internally into a JOIN. Sub-queries in the Where-statement SELECT moo FROM foo WHERE bar = (SELECT roger FROM wilco WHERE moo = me) However, if the sub-query has no reference to foo it can be optimized by the DB-server as static content and could be evaluated only once. Sub-queries in Select-Fields SELECT moo, (SELECT roger FROM wilco WHERE moo = me) AS bar FROM fooīe aware that a sub-query is executed for every resulting row from foo.Īvoid this if possible it may drastically slow down your query on huge datasets. In some cases a subquery helps optimizing a query, but a newer version of the DB-Server might make the optimization obsolete. On the other side, a modern DB-Server is able to optimize a lot. Especially if the sub-query is evaluated once or for every row! If you use sub-queries, you should be aware of how the DB-Server executes the sub-query. I hope I can provide one with this (very) late answer:įirst of all, let me say the most important: There are different forms of sub-queriesĪnd the second important statement: Size matters I've read multiple answers here some stated subqueries are faster, but it lacked a good explanation. When a subquery starts at the FROM clause (as opposed to the WHERE clause), the result set is referred to as a derived table (also known as a materialized subquery).In the year 2010 I would have joined the author of this questions and would have strongly voted for JOIN, but with much more experience (especially in MySQL) I can state: Yes subqueries can be better. Lastly, we select all records from the new table to verify that the correct data was inserted. We use a subquery (and a nested subquery) inside the INSERT statement to retrieve the data that we need, then insert it into the new table. In the above example, we first create a new table (for our data to be inserted into). SELECT first_name, last_name FROM actor a INSERT INTO ace_goldfinger_actors (first_name, last_name) Insert Data (from the following subqueries) (first_name VARCHAR(45), last_name VARCHAR(45)) You can use a subquery within any of the following statements: SELECT, INSERT, UPDATE, DELETE, SET, or DO.įor example, you could use a subquery inside an INSERT statement to insert data based on the results of a subquery.įor example, here's an INSERT statement containing a subquery and a nested subquery:ĭROP TABLE IF EXISTS ace_goldfinger_actors You can use a subquery anywhere an expression is allowed. Subqueries are not necessarily limited to the WHERE clause, or to the SELECT statement for that matter. So, now all we need to do is think of a film title and we can retrieve its actors - no need to try to remember the film ID. This happens to be the same film as in the previous example (its film_id is 2), therefore, we get the same list of actors as the previous example. The above example uses a nested subquery to retrieve the film_id from the film table where the title equals Ace Goldfinger. ![]()
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