WHERE vs HAVING

Why is it that you need to place columns you create yourself (for example "select 1 as number") after HAVING and not WHERE in MySQL?

WHERE is applied before GROUP BY, HAVING is applied after (and can filter on aggregates).

In general, you can reference aliases in neither of these clauses, but MySQL allows referencing SELECT level aliases in GROUP BY, ORDER BY and HAVING.

And are there any downsides instead of doing "WHERE 1" (writing the whole definition instead of a column name)

If your calculated expression does not contain any aggregates, putting it into the WHERE clause will most probably be more efficient.

All other answers on this question didn't hit upon the key point.

Assume we have a table:

CREATE TABLE `table` (
 `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
 `value` int(10) unsigned NOT NULL,
 PRIMARY KEY (`id`),
 KEY `value` (`value`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

And have 10 rows with both id and value from 1 to 10:

INSERT INTO `table`(`id`, `value`) VALUES (1, 1),(2, 2),(3, 3),(4, 4),(5, 5),(6, 6),(7, 7),(8, 8),(9, 9),(10, 10);

Try the following 2 queries:

SELECT `value` v FROM `table` WHERE `value`>5; -- Get 5 rows
SELECT `value` v FROM `table` HAVING `value`>5; -- Get 5 rows

You will get exactly the same results, you can see the HAVING clause can work without GROUP BY clause.

Here's the difference:

SELECT `value` v FROM `table` WHERE `v`>5;

The above query will raise error: Error #1054 - Unknown column 'v' in 'where clause'

SELECT `value` v FROM `table` HAVING `v`>5; -- Get 5 rows

WHERE clause allows a condition to use any table column, but it cannot use aliases or aggregate functions. HAVING clause allows a condition to use a selected (!) column, alias or an aggregate function.

This is because WHERE clause filters data before select, but HAVING clause filters resulting data after select.

So put the conditions in WHERE clause will be more efficient if you have many many rows in a table.

Try EXPLAIN to see the key difference:

EXPLAIN SELECT `value` v FROM `table` WHERE `value`>5;
+----+-------------+-------+-------+---------------+-------+---------+------+------+--------------------------+
| id | select_type | table | type  | possible_keys | key   | key_len | ref  | rows | Extra                    |
+----+-------------+-------+-------+---------------+-------+---------+------+------+--------------------------+
|  1 | SIMPLE      | table | range | value         | value | 4       | NULL |    5 | Using where; Using index |
+----+-------------+-------+-------+---------------+-------+---------+------+------+--------------------------+

EXPLAIN SELECT `value` v FROM `table` having `value`>5;
+----+-------------+-------+-------+---------------+-------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key   | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+-------+---------+------+------+-------------+
|  1 | SIMPLE      | table | index | NULL          | value | 4       | NULL |   10 | Using index |
+----+-------------+-------+-------+---------------+-------+---------+------+------+-------------+

You can see either WHERE or HAVING uses index, but the rows are different.

The main difference is that WHERE cannot be used on grouped item (such as SUM(number)) whereas HAVING can.

The reason is the WHERE is done before the grouping and HAVING is done after the grouping is done.