Question

Table layout:

CREATE TABLE t_order  (id INT, custId INT, order DATE) 

I’m looking for a SQL command to select a maximum of one row per order (the customer who owns the order is identified by a field named custId).

I want to select ONE of the customer’s orders (doesn’t matter which one, say sorted by id) if there is no order date given for any of the rows.

I want to retrieve an empty Resultset for the customerId, if there is already a record with given order date.

Here is an example. Per customer there should be one order at most (one without a date given). Orders that have already a date value should not appear at all.

   +---------------------------------------------------------+   |id    |    custId       |  date                          |   +---------------------------------------------------------+   | 1              10         NULL                          |   | 2              11         2008-11-11                    |   | 3              12         2008-10-23                    |   | 4              11         NULL                          |   | 5              13         NULL                          |   | 6              13         NULL                          |   +---------------------------------------------------------+                            |                            |                            |      Result                          \ |  /                           \  /   +---------------------------------------------------------+   |id    |    custId       |  date                          |   +---------------------------------------------------------+   | 1              10         NULL                          |   |                                                         |   |                                                         |   |                                                         |   | 5              13         NULL                          |   |                                                         |   +---------------------------------------------------------+                                 powered be JavE 

Edit: I’ve choosen glavić’s answer as the correct one, because it provides the correct result with slightly modified data:

   +---------------------------------------------------------+   |id    |    custId       |  date                          |   +---------------------------------------------------------+   | 1              10         NULL                          |   | 2              11         2008-11-11                    |   | 3              12         2008-10-23                    |   | 4              11         NULL                          |   | 5              13         NULL                          |   | 6              13         NULL                          |   | 7              11         NULL                          |   +---------------------------------------------------------+ 

Sfossen’s answer will not work when customers appear more than twice because of its where clause constraint a.id != b.id.

Quassnoi’s answer does not work for me, as I run server version 4.0.24 which yields the following error: alt text http://img25.imageshack.us/img25/8186/picture1vyj.png

Answer 1

Try this:

SELECT to1.* FROM t_order AS to1 WHERE     to1.date IS NULL AND      to1.custId NOT IN (         SELECT to2.custId         FROM t_order AS to2         WHERE to2.date IS NOT NULL         GROUP BY to2.custId     ) GROUP BY to1.custId 

For MySQL 4:

SELECT to1.* FROM t_order AS to1 LEFT JOIN t_order AS to2 ON     to2.custId = to1.custId AND     to2.date IS NOT NULL WHERE     to1.date IS NULL AND      to2.id IS NULL GROUP BY to1.custId