Chapter 6. Advanced SQL Queries
In the previous chapter, you saw how you can write queries that retrieve information from multiple tables. This chapter looks at more advanced types of queries. We will begin with relatively simple subqueries, which allow you to put one query inside another, rather than running two individual queries. Subqueries can be tied even more closely to the main query using a correlated subquery, where columns in the WHERE clause of the subquery directly reference columns in the main query.
Sometimes, you need to get similar information from more than one query, and there is some overlap between the results. You might not want to see the duplicates, or you might want to see only the results that two queries have in common. ...
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