How Can Correlated Subqueries Replace CROSS APPLY in MySQL?

CROSS/OUTER APPLY in MySQL

Despite being a widely used operator in T-SQL, MySQL does not natively support the CROSS APPLY syntax. This can present a challenge for developers who need to utilize CROSS APPLY's capabilities in MySQL environments.

Understanding CORRELATED SUBQUERIES

In MySQL, a common approach to approximating CROSS APPLY functionality is to employ correlated subqueries. A correlated subquery is a subquery that references columns from the outer query. By embedding a correlated subquery within a SELECT statement or join condition, you can achieve similar results to CROSS APPLY.

Example 1: Joining with Correlated Subquery as Predicate

Consider the following query, which attempts to use CROSS APPLY to retrieve a specific value from the ORD_HISTORY table:

SELECT ORD.ID
    ,ORD.NAME
    ,ORD.DATE
    ,ORD_HIST.VALUE
FROM ORD
CROSS APPLY (
    SELECT TOP 1 ORD_HISTORY.VALUE
    FROM ORD_HISTORY
    WHERE ORD.ID = ORD_HISTORY.ID
        AND ORD.DATE <= ORD_HISTORY.DATE
    ORDER BY ORD_HISTORY.DATE DESC
    ) ORD_HIST

Due to MySQL's lack of CROSS APPLY support, a more appropriate approach is to use a join with a correlated subquery as the predicate:

SELECT
   ORD.ID
  ,ORD.NAME
  ,ORD.DATE
  ,ORD_HISTORY.VALUE
FROM
  ORD
INNER JOIN
  ORD_HISTORY
    ON  ORD_HISTORY.ID
        =
        (SELECT ORD_HISTORY.ID
           FROM ORD_HISTORY
          WHERE ORD.ID = ORD_HISTORY.ID
            AND ORD.DATE <= ORD_HISTORY.DATE
       ORDER BY ORD_HISTORY.DATE DESC
          LIMIT 1
        )

In this query, the correlated subquery is used to fetch the appropriate value from ORD_HISTORY and embed it as the join condition.

Example 2: Using Correlated Subquery Directly in SELECT Statement

In cases where only a single field from the target table is required, the correlated subquery can be incorporated directly into the SELECT statement:

SELECT
   ORD.ID
  ,ORD.NAME
  ,ORD.DATE
  ,(SELECT ORD_HISTORY.VALUE
      FROM ORD_HISTORY
     WHERE ORD.ID = ORD_HISTORY.ID
       AND ORD.DATE <= ORD_HISTORY.DATE
  ORDER BY ORD_HISTORY.DATE DESC
     LIMIT 1
   )   AS VALUE
FROM
  ORD

By leveraging correlated subqueries, developers can effectively approximate CROSS APPLY functionality in MySQL, enabling them to perform complex data retrieval operations even in the absence of native CROSS APPLY support.

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