Practical SQL Handbook, The: Using SQL Variants, Fourth Edition

Summary

This chapter has explored subqueries, including the following:

• What subqueries are and where they appear

• How simple and correlated subqueries work

• Hints on when to use subqueries and when to stick with joins

• Subquery rules, restrictions, and limitations

After these preliminaries, we looked at operators:

• IN or NOT IN can return any number of rows

• Comparison operators (=, >, etc.) must return no more than one row

• EXISTS and NOT EXISTS test existence and are mostly used with correlated subqueries

Finally, we considered some advanced issues:

• Subqueries within subqueries—nested subqueries

• Subqueries in UPDATE, DELETE, and INSERT statements

• Subqueries in SELECT and FROM

Now that you can write plain and fancy queries using joins and subqueries, you ...