In addition to comparison operators and the IN membership test, MySQL also comes with a special EXISTS operator, designed specifically for use with subqueries. The EXISTS operator is used to test if a subquery generates any result rows, and executes the outer query only if it does.

  mysql> SELECT * FROM clients WHERE EXISTS (SELECT bid FROM

  branches_services GROUP BY bid HAVING COUNT(sid) >= 5);

  Empty set (0.17 sec)

  

In this case, since the subquery returns an empty result set - there are no branches using five or more services - the EXISTS test will return false and the outer query will not execute.

Look what happens when you doctor the subquery above to return something:

  mysql> SELECT * FROM clients WHERE EXISTS (SELECT bid FROM

  branches_services GROUP BY bid HAVING COUNT(sid) >= 4);

  +-----+-----------------------------+

  | cid | cname |

  +-----+-----------------------------+

  | 101 | JV Real Estate |

  | 102 | ABC Talent Agency |

  | 103 | DMW Trading |

  | 104 | Rabbit Foods Inc |

  | 110 | Sharp Eyes Detective Agency |

  +-----+-----------------------------+

  5 rows in set (0.27 sec)

  

In this case, since there are some branches using four or more services, the inner query will return a result set consisting of at least one row, the EXISTS test will return true, and the outer query will be executed.

It's important to note that the result set of the outer query above does
*not* list which customers have more than four services. Rather, it is simply a list of all customers, and it is returned only because the inner query generated a result set. In the example above, the result set generated by the inner query is itself immaterial; I could accomplish the same thing with the following query:

  mysql> SELECT * FROM clients WHERE EXISTS (SELECT 1);

  +-----+-----------------------------+

  | cid | cname |

  +-----+-----------------------------+

  | 101 | JV Real Estate |

  | 102 | ABC Talent Agency |

  | 103 | DMW Trading |

  | 104 | Rabbit Foods Inc |

  | 110 | Sharp Eyes Detective Agency |

  +-----+-----------------------------+

  5 rows in set (0.16 sec)

  

With this in mind, you might be wondering what use the EXISTS operator really is. Good question.

You see, the EXISTS operator is most often used in the context of what SQL gurus like to call an "outer reference" - that is, a reference within the subquery to a field in one or more of the queries enclosing it. When such an outer reference is used, MySQL has to run the subquery once for every record generated by the outer query, and therefore test the subquery as many times as there are records in the outer query's result set, since the inner query contains a reference to a field (whose value is different for every record) in the outer query.

An example might make this clearer. Consider the following, which re-runs the previous example with an outer reference to make the result set more
useful:

  mysql> SELECT bid, bdesc FROM branches WHERE EXISTS (SELECT bid FROM

  branches_services WHERE branches.bid = branches_services.bid GROUP BY bid HAVING 
  COUNT(sid) >= 4);

  +------+--------------+

  | bid | bdesc |

  +------+--------------+

  | 1011 | Corporate HQ |

  +------+--------------+

  1 row in set (0.22 sec)

  

And if you look at the data, you'll see that there is, in fact, only one branch using four or more services (branch ID 1011).

In this case, since the inner query contains a reference to a field in the outer query, MySQL cannot run the inner query just once (as it usually does). Rather, it has to run it over and over, once for every row in the outer table, then substitute the value of the named field from that row in the subquery, and then decide whether or not to include that outer row in the final result set on the basis of whether or not the corresponding subquery returns a result set or not.

Needless to say, this is expensive in terms of performance, and outer references should hence be avoided unless absolutely necessary. Usually, you can accomplish the same thing faster and more optimally with a join, as
below:

  mysql> SELECT branches.bid, branches.bdesc FROM branches_services, 

  mysql> branches

  WHERE branches_services.bid = branches.bid GROUP BY bid HAVING COUNT(sid)

  >= 4;

  +------+--------------+

  | bid | bdesc |

  +------+--------------+

  | 1011 | Corporate HQ |

  +------+--------------+

  1 row in set (0.28 sec)