Turn a Mysql Subquery in a Join

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How can I turn this subquery in a JOIN?

I read that subqueries are slower than JOINs.

SELECT 
    reklamation.id, 
    reklamation.titel,
    ( 
        SELECT reklamation_status.status 
        FROM reklamation_status 
        WHERE reklamation_status.id_reklamation = reklamation.id 
        ORDER BY reklamation_status.id DESC 
        LIMIT 1 
    ) as status
FROM reklamation 
WHERE reklamation.aktiv=1

Using JOIN query can be rewritten to:

SELECT reklamation.id, reklamation.titel, reklamation_status.status
FROM reklamation 
JOIN reklamation_status ON reklamation_status.id_reklamation = reklamation.id
WHERE reklamation.aktiv=1

Turn a Mysql Subquery in a Join, Using JOIN query can be rewritten to: SELECT reklamation.id, reklamation.titel, reklamation_status.status FROM reklamation JOIN� Now, with the help of followings steps we can convert the above subquery into inner join − 1. Move the ‘Reserve’ table named in the subquery to the FROM clause. 2. The WHERE clause compares the customer_id column to the ids returned from the subquery. Hence convert the expression to an explicit direct comparison between id columns of two

This should do it:

SELECT r.id, r.titel, MAX(s.id) as status
FROM reklamation r
LEFT JOIN reklamation_status s ON s.id_reklamation = r.id
WHERE r.aktiv = 1   
GROUP BY r.id, r.titel

The key point here is to use aggregation to manage the cardinality between reklamation and reklamation_status. In your original code, the inline subquery uses ORDER BY reklamation_status.id DESC LIMIT 1 to return the highest id in reklamation_status that corresponds to the current reklamation. Without aggregation, we would probably get multiple records in the resultset for each reklamation (one for each corresponding reklamation_status).

Another thing is to consider is the type of the JOIN. INNER JOIN would filter out records of reklamations that do not have a reklamation_status: the original query with the inline subquery does not behave like that, so I chose LEFT JOIN. If you can guarantee that every reklamation has at least one child in reklamation_status, you can safely switch back to INNER JOIN (which might perform more efficiently).


PS:

I read that subqueries are slower than JOINs.

This is not a universal truth. It depends on many factors and cannot be told without seeing your exact use case.

rewriting-subqueries, Please update any bookmarks that point to the old page. HOME NEXT. Related Documentation. A subquery can be used with JOIN operation In the example below, the subquery actually returns a temporary table which is handled by database server in memory. The temporary table from the subquery is given an alias so that we can refer to it in the outer select statement.

What you read is incorrect. Subqueries can be slower, faster, or the same as joins. I would write the query as:

SELECT r.id, r.titel,
       (SELECT rs.status 
        FROM reklamation_status rs
        WHERE rs.id_reklamation = r.id 
        ORDER BY rs.id DESC 
        LIMIT 1 
       ) as status
FROM reklamation  r
WHERE r.aktiv = 1;

For performance, you want an index on reklamation_status(id_reklamation, id, status).

By the way, this is a case where the subquery is probably the fastest method for expressing this query. If you attempt a JOIN, then you need some sort of aggregation to get the most recent status. That can be expensive.

13.2.10.11 Rewriting Subqueries as Joins, Also, on some occasions, it is not only possible to rewrite a query without a subquery, but it can be more efficient to make use of some of these techniques rather� A MySQL subquery is a query nested within another query such as SELECT, INSERT, UPDATE or DELETE. In addition, a subquery can be nested inside another subquery. A MySQL subquery is called an inner query while the query that contains the subquery is called an outer query. A subquery can be used anywhere that expression is used and must be closed

MySQL 8.0 Reference Manual :: 13.2.11 Subqueries, Many people find subqueries more readable than complex joins or unions. Indeed, it was the innovation of subqueries that gave people the original idea of� A subquery can return a scalar (a single value), a single row, a single column, or a table (one or more rows of one or more columns). These are called scalar, column, row, and table subqueries. MySQL Subquery Example: Using a subquery, list the name of the employees, paid more than 'Alexander' from emp_details .

8.2.2.3 Optimizing Subqueries with the EXISTS Strategy, Most optimizations cannot deal with predicates that may be turned on and off at query Table-condition generator: If the subquery is a join of several tables, the � Knowing about a subquery versus inner join can help you with interview questions and performance issues. Though subqueries have unique abilities, there are times when it is better to use other SQL constructs such as joins. By reading this article you’ll learn about several types of subqueries, and how each can be switched to another form

How can we convert subqueries to INNER JOIN?, Move the 'Reserve' table named in the subquery to the FROM clause. The WHERE clause compares the customer_id column to the ids returned from the subquery. Subqueries allow you to use the results of another query in the outer query. In some cases, subqueries can replace complex joins and unions. Disadvantages of Subquery: The optimizer is more mature for MYSQL for joins than for subqueries, so in many cases a statement that uses a subquery can be executed more efficiently if you rewrite it as join.