How to use distinct and group by in MySQL

Let’s talk about the general conclusion first:

• When the semantics are the same and there is an index: group by## Both # and distinct can use indexes with the same efficiency.

• With the same semantics and no index:

  distinct is more efficient than group by. The reason is that both distinct and group by will perform grouping operations, but group by may perform sorting and trigger filesort, resulting in low SQL execution efficiency.

Based on this conclusion, you may ask:

• Why

  group by# when the semantics are the same and there is an index? ## and distinct have the same efficiency?

Under what circumstances will
• group by

  perform a sorting operation?

Find answers to these two questions. Next, let’s take a look at the basic use of

distinct

and group by. Usage of distinct

Usage of distinct

SELECT DISTINCT columns FROM table_name WHERE where_conditions;

For example:

mysql> select distinct age from student;
+------+
| age  |
+------+
|   10 |
|   12 |
|   11 |
| NULL |
+------+
4 rows in set (0.01 sec)

DISTINCT

The keyword is used to return uniquely different values. It is used before the first field in the query statement and applies to all columns in the main clause. If a column has a NULL value and you use the

DISTINCT

clause on the column, MySQL will retain one NULL value and delete the other NULL values ​​because the DISTINCT clause statement treats all NULL values ​​as the same value. distinct Multi-column deduplication

distinct

Multi-column deduplication is performed based on the specified deduplication column information, that is, only all specified column information If they are all the same, it will be considered as duplicate information. <pre class="brush:sql;">SELECT DISTINCT column1,column2 FROM table_name WHERE where_conditions; mysql&gt; select distinct sex,age from student; +--------+------+ | sex | age | +--------+------+ | male | 10 | | female | 12 | | male | 11 | | male | NULL | | female | 11 | +--------+------+ 5 rows in set (0.02 sec)</pre>Usage of group by

For basic deduplication, the use of

group by

is similar to distinct. Single column deduplication

Syntax:

SELECT columns FROM table_name WHERE where_conditions GROUP BY columns;

Execution:

mysql> select age from student group by age;
+------+
| age  |
+------+
|   10 |
|   12 |
|   11 |
| NULL |
+------+
4 rows in set (0.02 sec)

Multiple column deduplication

Syntax:

SELECT columns FROM table_name WHERE where_conditions GROUP BY columns;

Execution:

mysql> select sex,age from student group by sex,age;
+--------+------+
| sex    | age  |
+--------+------+
| male   |   10 |
| female |   12 |
| male   |   11 |
| male   | NULL |
| female |   11 |
+--------+------+
5 rows in set (0.03 sec)

Difference example

The syntax difference between the two is that

group by

can perform single-column deduplication, and the principle of group by The results are grouped and sorted first, and then the first piece of data in each group is returned. And deduplication is performed based on the fields following group by. For example:

mysql> select sex,age from student group by sex;
+--------+-----+
| sex    | age |
+--------+-----+
| male   |  10 |
| female |  12 |
+--------+-----+
2 rows in set (0.03 sec)

distinct and group by principle

In most examples,

DISTINCT

can be regarded as a special GROUP BY, their implementation is based on grouping operations, and they can all be implemented through loose index scan and compact index scan (the content of index scan will be introduced in detail in other articles, so I will not introduce it in detail here).

DISTINCT

and GROUP BY can both be scanned and searched using indexes. For example, the following two SQLs (just look at the content of the extra at the end of the table), we analyze these two SQLs, we can see that in the extra, these two SQLs use compact index scanningUsing index for group -by. So, in general, for

DISTINCT

and GROUP BY statements with the same semantics, we can use the same index optimization methods to optimize them. <pre class="brush:sql;">mysql&gt; explain select int1_index from test_distinct_groupby group by int1_index; +----+-------------+-----------------------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-----------------------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+ | 1 | SIMPLE | test_distinct_groupby | NULL | range | index_1 | index_1 | 5 | NULL | 955 | 100.00 | Using index for group-by | +----+-------------+-----------------------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+ 1 row in set (0.05 sec) mysql&gt; explain select distinct int1_index from test_distinct_groupby; +----+-------------+-----------------------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-----------------------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+ | 1 | SIMPLE | test_distinct_groupby | NULL | range | index_1 | index_1 | 5 | NULL | 955 | 100.00 | Using index for group-by | +----+-------------+-----------------------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+ 1 row in set (0.05 sec)</pre>But for

GROUP BY

, before MYSQL8.0, GROUP Y will be implicitly sorted by fields by default. As you can see, the following sql statement uses a temporary table and also performs filesort.

mysql> explain select int6_bigger_random from test_distinct_groupby GROUP BY int6_bigger_random;
+----+-------------+-----------------------+------------+------+---------------+------+---------+------+-------+----------+---------------------------------+
| id | select_type | table                 | partitions | type | possible_keys | key  | key_len | ref  | rows  | filtered | Extra                           |
+----+-------------+-----------------------+------------+------+---------------+------+---------+------+-------+----------+---------------------------------+
|  1 | SIMPLE      | test_distinct_groupby | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 97402 |   100.00 | Using temporary; Using filesort |
+----+-------------+-----------------------+------------+------+---------------+------+---------+------+-------+----------+---------------------------------+
1 row in set (0.04 sec)

Implicit sorting

For implicit sorting, we can refer to MySQL’s official explanation:

https://dev.mysql.com/doc/refman/5.7 /en/order-by-optimization.html

GROUP BY implicitly sorts by default (that is, in the absence of ASC or DESC designators for GROUP BY columns). However, relying on implicit GROUP BY sorting (that is, sorting in the absence of ASC or DESC designators) or explicit sorting for GROUP BY (that is, by using explicit ASC or DESC designators for GROUP BY columns) is deprecated. To produce a given sort order, provide an ORDER BY clause.

Broad explanation:

GROUP BY defaults to implicit sorting (meaning that sorting will also be performed even if the GROUP BY column does not have an ASC or DESC indicator). However, GROUP BY for explicit or implicit sorting is deprecated. To generate a given sort order, provide an ORDER BY clause.

So, before MySQL8.0,

GROUP BY

will sort the results according to the effect field (the subsequent field of GROUP BY) by default. When the index can be used, GROUP BY does not require additional sorting operations; but when the index cannot be used for sorting, the MySQL optimizer has to choose to use a temporary table and then sort itGROUP BY. And when the size of the result set exceeds the temporary table size set by the system, MySQL will copy the temporary table data to the disk before operating, and the execution efficiency of the statement will become extremely low. This is why MySQL has chosen to deprecate this operation (implicit sorting).

Based on the above reasons, Mysql has optimized and updated this in 8.0:

https://dev.mysql.com/doc/refman/8.0/en/order-by-optimization.html

Previously (MySQL 5.7 and lower), GROUP BY sorted implicitly under certain conditions. In MySQL 8.0, that no longer occurs, so specifying ORDER BY NULL at the end to suppress implicit sorting (as was done previously) is no longer necessary. However, query results may differ from previous MySQL versions. To produce a given sort order, provide an ORDER BY clause.

A rough explanation:

In the past (before MySQL5.7 version), Group by would perform implicit sorting based on certain conditions. In MySQL 8.0, this functionality has been removed, so it is no longer necessary to disable implicit ordering by adding order by null, however, query results may differ from previous MySQL versions. To produce results in a given order, specify the fields to be sorted by ORDER BY.

Therefore, our conclusion also comes out:

• In the case of the same semantics and index: group by and distinct Both can use indexes and have the same efficiency. Because group by and distinct are nearly equivalent, distinct can be regarded as a special group by.

• In the case of the same semantics and no index: distinct is more efficient than group by. The reason is that both distinct and group by will perform grouping operations, but group by will perform implicit sorting before MySQL8.0, causing filesort to be triggered and sql execution efficiency low. However, starting from MySQL8.0, MySQL has deleted the implicit sorting. Therefore, at this time, with the same semantics and no index, the execution efficiency of group by and distinct is almost the same. equivalent.

Compared with distinct, group by has clear semantics. And since the distinct keyword will take effect on all fields, group by is more flexible when performing composite business processing. group by can update the data according to the grouping situation. For complex processing, such as filtering data through having, or operating on data through aggregate functions.

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