WHERE vs HAVING

Question

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

Answer 1

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.

Answer 2

Having is only used with aggregation but where with non aggregation statements If you have where word put it before aggregation (group by)

Answer 3

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;

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.

Answer 4

WHERE filters before data is grouped, and HAVING filters after data is grouped. This is an important distinction; rows that are eliminated by a WHERE clause will not be included in the group. This could change the calculated values which, in turn(=as a result) could affect which groups are filtered based on the use of those values in the HAVING clause.

And continues,

HAVING is so similar to WHERE that most DBMSs treat them as the same thing if no GROUP BY is specified. Nevertheless, you should make that distinction yourself. Use HAVING only in conjunction with GROUP BY clauses. Use WHERE for standard row-level filtering.

Excerpt From: Forta, Ben. “Sams Teach Yourself SQL in 10 Minutes (5th Edition) (Sams Teach Yourself...).”.

Answer 5

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.

Answer 6

HAVING is used to filter on aggregations in your GROUP BY.

For example, to check for duplicate names:

SELECT Name FROM Usernames
GROUP BY Name
HAVING COUNT(*) > 1