MySQL :: MySQL 8.0 Reference Manual :: 5.1.11 Server SQL Modes https://dev.mysql.com/doc/refman/8.0/en/sql-mode.html#sqlmode_only_full_group_by
Reject queries for which the select list, HAVING
condition, or ORDER BY
list refer to nonaggregated columns that are neither named in the GROUP BY
clause nor are functionally dependent on (uniquely determined by) GROUP BY
columns.
A MySQL extension to standard SQL permits references in the HAVING
clause to aliased expressions in the select list. The HAVING
clause can refer to aliases regardless of whether ONLY_FULL_GROUP_BY
is enabled.
For additional discussion and examples, see Section 12.20.3, “MySQL Handling of GROUP BY”.
12.20.3 MySQL Handling of GROUP BY
SQL-92 and earlier does not permit queries for which the select list, HAVING
condition, or ORDER BY
list refer to nonaggregated columns that are not named in the GROUP BY
clause. For example, this query is illegal in standard SQL-92 because the nonaggregated name
column in the select list does not appear in the GROUP BY
:
SELECT o.custid, c.name, MAX(o.payment)
FROM orders AS o, customers AS c
WHERE o.custid = c.custid
GROUP BY o.custid;
For the query to be legal in SQL-92, the name
column must be omitted from the select list or named in the GROUP BY
clause.
SQL:1999 and later permits such nonaggregates per optional feature T301 if they are functionally dependent on GROUP BY
columns: If such a relationship exists between name
and custid
, the query is legal. This would be the case, for example, were custid
a primary key of customers
.
MySQL implements detection of functional dependence. If the ONLY_FULL_GROUP_BY
SQL mode is enabled (which it is by default), MySQL rejects queries for which the select list, HAVING
condition, or ORDER BY
list refer to nonaggregated columns that are neither named in the GROUP BY
clause nor are functionally dependent on them.
If ONLY_FULL_GROUP_BY
is disabled, a MySQL extension to the standard SQL use of GROUP BY
permits the select list, HAVING
condition, or ORDER BY
list to refer to nonaggregated columns even if the columns are not functionally dependent on GROUP BY
columns. This causes MySQL to accept the preceding query. In this case, the server is free to choose any value from each group, so unless they are the same, the values chosen are nondeterministic, which is probably not what you want. Furthermore, the selection of values from each group cannot be influenced by adding an ORDER BY
clause. Result set sorting occurs after values have been chosen, and ORDER BY
does not affect which value within each group the server chooses. Disabling ONLY_FULL_GROUP_BY
is useful primarily when you know that, due to some property of the data, all values in each nonaggregated column not named in the GROUP BY
are the same for each group.
You can achieve the same effect without disabling ONLY_FULL_GROUP_BY
by using ANY_VALUE()
to refer to the nonaggregated column.
The following discussion demonstrates functional dependence, the error message MySQL produces when functional dependence is absent, and ways of causing MySQL to accept a query in the absence of functional dependence.
This query might be invalid with ONLY_FULL_GROUP_BY
enabled because the nonaggregated address
column in the select list is not named in the GROUP BY
clause:
SELECT name, address, MAX(age) FROM t GROUP BY name;
The query is valid if name
is a primary key of t
or is a unique NOT NULL
column. In such cases, MySQL recognizes that the selected column is functionally dependent on a grouping column. For example, if name
is a primary key, its value determines the value of address
because each group has only one value of the primary key and thus only one row. As a result, there is no randomness in the choice of address
value in a group and no need to reject the query.
SELECT @@sql_mode;
ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_ENGINE_SUBSTITUTION
SHOW GLOBAL VARIABLES LIKE "%mode%";
Variable_name Value
block_encryption_mode aes-128-ecb
gtid_mode OFF
innodb_autoinc_lock_mode 2
innodb_strict_mode ON
offline_mode OFF
rbr_exec_mode STRICT
slave_exec_mode STRICT
sql_mode ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_ENGINE_SUBSTITUTION
ssl_fips_mode OFF
SELECT * FROM performance_schema.global_variables WHERE VARIABLE_NAME="sql_mode";
查找全局变量的3种sql语句;
User variables are written as @
, where the variable name var_name
var_name
consists of alphanumeric characters, .
, _
, and $
. A user variable name can contain other characters if you quote it as a string or identifier (for example, @'my-var'
, @"my-var"
, or @`my-var`
).