My Books

Text Processing with JavaScript

Solve complex text validation, extraction, and modification problems efficiently in JavaScript.

Modern Async JavaScript

Delve into async features of JavaScript from ES2020 through ESNext. Start building custom asynchronous iterators and generators, and more for fast, lean code.

JavaScript Brain Teasers

Ready to test your JavaScript skills? Challenge yourself with these brain-teasing puzzles, supercharge your learning journey, and emerge as a JavaScript pro.

Coding

SQL – Subquery Example

Faraz

In some cases you can do things with a subquery that can't be done with join. However when using subqueries, there can be performance costs.

An SQL subquery is a SELECT statement inside another statement. Subqueries are useful alternates to JOIN and can, in some cases, do things that JOIN cannot. Here is an example of SQL subquery:

SELECT f_name, l_name FROM employee WHERE id IN (SELECT id FROM salary WHERE salary < 70000);

In this example, (SELECT id FROM salary WHERE salary < 70000) is a subquery, you can even nest subqueries within another subqueries.

However, subqueries are not as quick as joins when selecting big datasets and there can be surprising performance costs. Please note that the value that subquery returns must mach the value that the parent query expects. For example, if you use WHERE id = 52 then value must be a single value, not a list, otherwise you will encounter an error.

Lean More:
MySQL 5.0 Reference Manual :: 13.2.9 Subquery Syntax

Have your say