the ONLY_FULL_GROUP_BY SQL mode is enabled (which it is by default) 用户变量

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

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 BYclause 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 @var_name, where the variable 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`).