Appendix B. MySQL Extensions to the SQL Language
Since this book uses the MySQL server for all of the examples, I thought it would be useful for those readers planning to continue using MySQL to include an Appendix on MySQL’s extensions
to the SQL language. This appendix explores some of MySQL’s extensions to the
delete statements that can be very useful in certain situations.
MySQL’s implementation of the
select statement includes two additional clauses, which are discussed in the following subsections.
The limit Clause
In some situations, you may not be interested in all of the rows returned by a query. For example, you might construct a query that returns all of the bank tellers along with the number of accounts opened by each teller. If your reason for executing the query is to determine the top three tellers so that they can receive an award from the bank, then you don’t necessarily need to know who came in fourth, fifth, etc. To help with these types of situations, MySQL’s
select statement includes the
limit clause, which allows you to restrict the number of rows returned by a query.
To demonstrate the utility of the
limit clause, I will begin by constructing a query to show the number of accounts opened by each bank teller:
SELECT open_emp_id, COUNT(*) how_many->
GROUP BY open_emp_id;+-------------+----------+ | open_emp_id | how_many | +-------------+----------+ | 1 | 8 | | 10 | 7 | | 13 | 3 | | 16 | 6 | ...