Diving into Advanced SQL Topics
Haythem Balti, Kim Weiss · 2023
This chapter helps the reader to use a simple subquery and uses optimization techniques to improve the performance of a MySQL database. It explains the pros and cons of views and the requirements of a database transaction, and describe the use of indexes to improve database performance. SQL is a specialized language that executes only in the context of a relational database. A subquery is a syntactically correct, complete query that is embedded in another query to produce a value or tabular result set. WorkerId values could be grabbed with a query that is within an IN clause, and the resulting values from that query would be used for the IN clause. A secondary SELECT can be built on top of a subquery, or a subquery can be JOINed to a table. A subquery can even be JOINed to another subquery.