Can You Combine Multiple SELECT Statements into a Single Query in PHP (MySQL)?

Performing Multiple Select Statements in a Single Query

In PHP (MySQL), it is possible to execute multiple select statements in a single query to retrieve data from various tables. This approach can be beneficial in scenarios where multiple counts or aggregate values are required from different tables.

Consider the following example, where counts are needed from three tables: user_table, cat_table, and course_table. Traditionally, these counts would be obtained using separate select statements as follows:

SELECT COUNT(id) AS tot_user FROM user_table
SELECT COUNT(id) AS tot_cat FROM cat_table
SELECT COUNT(id) AS tot_course FROM course_table

However, it is possible to optimize this process by combining all select statements into a single query. This can be achieved using subqueries within a single SELECT statement:

SELECT  (
    SELECT COUNT(*)
    FROM   user_table
) AS tot_user,
(
    SELECT COUNT(*)
    FROM   cat_table
) AS tot_cat,
(
    SELECT COUNT(*)
    FROM   course_table
) AS tot_course

In this combined query, the subqueries enclosed within parentheses retrieve the counts from each table. The outer SELECT statement then assigns these counts to aliases (tot_user, tot_cat, and tot_course).

As long as the number of subqueries remains relatively low, combining select statements in this manner generally does not impact performance negatively. Database optimizations, such as query caching and indexing, can mitigate any potential slowdown caused by the additional subqueries.

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