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https://dev.mysql.com/doc/refman/5.7/en/sys-processlist.html
db The default database for the thread, or NULL if there is none. current_statement The statement the thread is executing, or NULL if it is not executing any statement. The MySQL process list indicates the operations currently being performed by ...
https://dev.mysql.com/doc/refman/5.7/en/sys-ps-is-thread-instrumented.html
Returns YES or NO to indicate whether Performance Schema instrumentation for a given connection ID is enabled, UNKNOWN if the ID is unknown, or NULL if the ID is NULL. This is a value of the type given in the PROCESSLIST_ID column of the ...
https://dev.mysql.com/doc/refman/5.7/en/thread-information.html
row *************************** Id: 1 User: event_scheduler Host: localhost db: NULL Command: Daemon Time: 2756681 State: Waiting on empty queue Info: NULL *************************** 2. To ascertain what your MySQL server is doing, it can be ...
https://dev.mysql.com/doc/refman/5.7/en/user-variables.html
User variables can be assigned a value from a limited set of data types: integer, decimal, floating-point, binary or nonbinary string, or NULL value. If you refer to a variable that has not been initialized, it has a value of NULL and a type of ...
https://dev.mysql.com/doc/refman/5.7/en/using-date.html
When a '0000-00-00' date is used through Connector/ODBC, it is automatically converted to NULL because ODBC cannot handle that kind of date. MySQL permits you to store a “zero” value of '0000-00-00' as a “dummy date.” This is in some cases ...You should use this format in UPDATE expressions and in the WHERE clause of SELECT ...
https://dev.mysql.com/doc/refman/5.7/en/version-tokens-usage.html
Otherwise, an error occurs: mysql> SELECT version_tokens_lock_shared('lock1', 'lock2', 0); +-------------------------------------------------+ | version_tokens_lock_shared('lock1', 'lock2', 0) | +-------------------------------------------------+ | ... Before using Version Tokens, install it according to the instructions provided at Section 5.5.5.2, “Installing or Uninstalling Version ...
https://dev.mysql.com/doc/refman/5.7/en/where-optimization.html
This is also done for any NOT NULL expression when used with only one table. A table that is used with a WHERE clause on a PRIMARY KEY or a UNIQUE index, where all index parts are compared to constant expressions and are defined as NOT NULL. This ...
https://dev.mysql.com/doc/refman/5.7/en/charset-collation-implementations.html
Example: "U+0000 NULL" does not have a weight and is ignorable. MySQL implements several types of collations: Simple collations for 8-bit character sets This kind of collation is implemented using an array of 256 weights that defines a one-to-one ...
https://dev.mysql.com/doc/refman/5.7/en/charset-conversion.html
For example, if you specified NOT NULL and an explicit DEFAULT value, you should also provide them in the ALTER TABLE statement. To convert a binary or nonbinary string column to use a particular character set, use ALTER TABLE. For successful ...
https://dev.mysql.com/doc/refman/5.7/en/charset-errors.html
If character_set_results has a value of NULL or binary, no conversion occurs. This section describes how the MySQL server uses character sets for constructing error messages. For information about the language of error messages (rather than the ...