Session variables affect its operation for individual client connections. A given system variable can have both a global and a session value. Global and session system variables are related as follows:. When the server starts, it initializes each global variable to its default value. These defaults can be changed by options specified on the command line or in an option file. See Section 4. The server also maintains a set of session variables for each client that connects.
The client's session variables are initialized at connect time using the current values of the corresponding global variables. For some system variables, the session value is not initialized from the corresponding global value; if so, that is indicated in the variable description.
System variable values can be set globally at server startup by using options on the command line or in an option file. At startup, the syntax for system variables is the same as for command options, so within variable names, dashes and underscores may be used interchangeably.
When you use a startup option to set a variable that takes a numeric value, the value can be given with a suffix of K , M , or G either uppercase or lowercase to indicate a multiplier of , 2 or 3 ; that is, units of kilobytes, megabytes, or gigabytes, respectively.
As of MySQL 8. Thus, the following command starts the server with an InnoDB log file size of 16 megabytes and a maximum packet size of one gigabyte:. The lettercase of suffix letters does not matter; 16M and 16m are equivalent, as are 1G and 1g. Many system variables are dynamic and can be changed at runtime by using the SET statement.
For a list, see Section 5. To change a system variable with SET , refer to it by name, optionally preceded by a modifier. At runtime, system variable names must be written using underscores, not dashes. The following examples briefly illustrate this syntax:. Persist a global system variable to the mysqld-auto. For complete details about SET syntax, see Section For a description of the privilege requirements for setting and persisting system variables, see Section 5.
Suffixes for specifying a value multiplier can be used when setting a variable at server startup, but not to set the value with SET at runtime. On the other hand, with SET you can assign a variable's value using an expression, which is not true when you set a variable at server startup. For example, the first of the following lines is legal at server startup, but the second is not:. Conversely, the second of the following lines is legal at runtime, but the first is not:.
With a LIKE clause, the statement displays only those variables that match the pattern. To obtain a specific variable name, use a LIKE clause as shown:. Wildcard characters can be used in any position within the pattern to be matched. In practice, this is rarely necessary. Server Configuration Defaults.
Server Configuration Validation. Server System Variable Reference. Posted by: aband aband Date: May 13, PM in following query , abc is a bug id, the component associated with abc is "star", there are 5 bugs in jira project which has component "star". Posted: 3 days ago A SET statement that assigns variable values is not written to the binary log, so in replication scenarios it affects only the host on which you execute it. To affect all replication hosts, execute the statement on each host. The following sections describe SET syntax for set ting variables.
The query to create a stored procedure is as follows. Trigger to set multiple values in variable. Posted: 1 week ago A SET is a string object that can have zero or more values, each of which must be chosen from a list of permitted values specified when the table is created. SET column values that consist of multiple set members are specified …. Posted: 4 days ago A SET statement that assigns variable values is not written to the binary log, so in replication scenarios it affects only the host on which you execute it.
You can initialize many variables at once, separating each assignment statement with a comma, like this: mysql Copy. Posted: 5 days ago A SET statement can contain multiple variable assignments, separated by commas.
When you need to retrieve a single row from a table or query, you can use the following syntax in SQL Server:. FROM Articles. Posted: 5 days ago Example - Declaring a variable. You can then later set or change the value of the vSite variable, as follows:. If there is no symbol, that would mean it is a local This provides a way to save a result returned from one query, then refer to it later in other queries.
User variables can be assigned a value from a limited set of data types: integer, decimal, floating-point, binary or nonbinary string, or NULL In this blog post, we will explain the differences in managing the configuration variables between MySQL 5. Suppose you wish to update a table named items which is defined using the statement shown here:. As shown here, this statement does not work:. Instead, you can employ a multi-table update in which the subquery is moved into the list of tables to be updated, using an alias to reference it in the outermost WHERE clause, like this:.
Because the optimizer tries by default to merge the derived table discounted into the outermost query block, this works only if you force materialization of the derived table. In this case, the subquery is materialized by default rather than merged, so it is not necessary to disable merging of the derived table. Data Definition Statements. LIKE Statement. Silent Column Specification Changes. Secondary Indexes and Generated Columns.
Data Manipulation Statements. The Subquery as Scalar Operand. Comparisons Using Subqueries. Rewriting Subqueries as Joins.
Restrictions on Subqueries. Transactional and Locking Statements. Statements That Cause an Implicit Commit. Restrictions on XA Transactions. Replication Statements. Variables in Stored Programs.
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