Subqueries
SQL, as we have seen, has just a few types of DML statements and each of these has several optional clauses. This, I hope you've found, makes it easy to learn the basics. At the same time, the number of combinations of options that are possible can be overwhelming. This is one of the things that make SQL such a powerful and expressive language. One combination is especially formidable: the subquery, which is a SELECT statement within another SQL statement.
Subqueries are important in a relational database for the same reason that joins are. Because we store data in normalized tables, we frequently have to combine data from multiple tables or use data from one table to determine what data to use, change, or delete in another table. ...
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