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?

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

Oof

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.

Aeroembolism

Answer 2

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.

Tarbes

I appreciate that you mentioned EXPLAIN! – Wagstaff

Because of HAVING clause filters data after select, WHERE clause will be more effective. So if this is true, when must we use HAVING instead of WHERE? – Bartz

@Bartz In case you have to filter data after select, you need HAVING clause, usually we use it with GROUP BY clause, e.g.: SELECT value, COUNT(*) frequency FROM table GROUP BY value HAVING frequency > 10 – Tarbes

@Fishdrowned, Why does MySQL even allow non-aggregate predicates in having in the first place? – Martensite

Excellent post. A couple of suggested clarifications: Change ...HAVING clause can use both column and alias. to ...HAVING clause can use either column or alias. and change ...WHERE clause will be more effective to ...WHERE clause will be more efficient – Loesceke

The HAVING clause was added to SQL because the WHERE keyword could not be used with aggregate functions. – Rochellerochemont

I'm confused which row you are referring in your last statement. Can you clarify? – Curling

This answer does not even begin to explain WHERE vs HAVING - It just shows the non-standard behaviour of MySQL when one uses HAVING incorrectly. – Food

Answer 3

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.

Kareenkarel

Answer 4

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

Joselynjoseph

It's true to some extant. You can still put your whole 'Where' in the having clause. – Kareenkarel

Also see #2905792 – Martensite

Answer 5

These 2 will be feel same as first as both are used to say about a condition to filter data. Though we can use ‘having’ in place of ‘where’ in any case, there are instances when we can’t use ‘where’ instead of ‘having’. This is because in a select query, ‘where’ filters data before ‘select’ while ‘having’ filter data after ‘select’. So, when we use alias names that are not actually in the database, ‘where’ can’t identify them but ‘having’ can.

Ex: let the table Student contain student_id,name, birthday,address.Assume birthday is of type date.

SELECT * FROM Student WHERE YEAR(birthday)>1993; /*this will work as birthday is in database.if we use having in place of where too this will work*/

SELECT student_id,(YEAR(CurDate())-YEAR(birthday)) AS Age FROM Student HAVING Age>20; 
/*this will not work if we use ‘where’ here, ‘where’ don’t know about age as age is defined in select part.*/

Lacteous

This real life example clarifies fully the difference between WHERE and HAVING. – Abroach

Clearly shows the difference between Having and where. Thanks. – Donovan

Answer 6

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...).”.

Unspotted

Answer 7

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

Folacin