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https://dev.mysql.com/doc/refman/5.7/en/subquery-errors.html
Consider the following example: SELECT * FROM t1 WHERE column1 = (SELECT column1 FROM t2); If SELECT column1 FROM t2 returns just one row, the previous query works. Incorrect number of rows from subquery: ERROR 1242 (ER_SUBSELECT_NO_1_ROW) SQLSTATE ...
https://dev.mysql.com/doc/refman/5.7/en/table-locking.html
MySQL uses table locking (instead of page, row, or column locking) for all storage engines except InnoDB. Splitting table contents into separate tables may help, by allowing queries to run against columns in one table, while updates are confined to ... InnoDB tables use row-level locking so that multiple sessions and applications can read from and write to the same table simultaneously, without making each other wait or producing inconsistent ...
https://dev.mysql.com/doc/refman/5.7/en/user-variables.html
(This is the same coercibility as for table column values.) Hexadecimal or bit values assigned to user variables are treated as binary strings. The following example illustrates this: mysql> SET @a='test'; mysql> SELECT @a,(@a:=20) FROM tbl_name; ...
https://dev.mysql.com/doc/refman/5.7/en/using-explain.html
Note EXPLAIN can also be used to obtain information about the columns in a table. EXPLAIN tbl_name is synonymous with DESCRIBE tbl_name and SHOW COLUMNS FROM tbl_name. For more information, see Section 13.8.1, “DESCRIBE Statement”, and Section ... The EXPLAIN statement provides information about how MySQL executes statements: EXPLAIN works with SELECT, DELETE, INSERT, REPLACE, and UPDATE ...
https://dev.mysql.com/doc/refman/5.7/en/adding-collation-choosing-id.html
The collation ID that you choose appears in these contexts: The ID column of the Information Schema COLLATIONS table. To add a collation, you must choose an ID value that is not currently used. The range of IDs from 1024 to 2047 is reserved for ...
https://dev.mysql.com/doc/refman/5.7/en/alter-table-problems.html
If you get a duplicate-key error when using ALTER TABLE to change the character set or collation of a character column, the cause is either that the new column collation maps two keys to the same value or that the table is corrupted. In the latter ...
https://dev.mysql.com/doc/refman/5.7/en/ansi-diff-update.html
If you access a column from the table to be updated in an expression, UPDATE uses the current value of the column. The second assignment in the following statement sets col2 to the current (updated) col1 value, not the original col1 value. The ...
https://dev.mysql.com/doc/refman/5.7/en/assigning-passwords.html
Required credentials for clients that connect to the MySQL server can include a password. MySQL stores credentials in the user table in the mysql system database. Operations that assign or modify passwords are permitted only to users with the ...
https://dev.mysql.com/doc/refman/5.7/en/change-master-to.html
The address configured with this option, if any, can be seen in the Master_Bind column of the output from SHOW SLAVE STATUS. If you are using a table for the source metadata repository (server started with master_info_repository=TABLE), the value ...
https://dev.mysql.com/doc/refman/5.7/en/charset-errors.html
Parameters in the message template are replaced with values that apply to a specific error occurrence: Identifiers such as table or column names use UTF-8 internally so they are copied as is. This section describes how the MySQL server uses ...