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The leading +  or -  character and trailing list of modifiers are used for flag characters such as d  or f  that can enable a debug operation for all applicable modifiers or just some of them: 

•With no leading +  or - , the flag value is set to exactly the modifier list as given. 

•With a leading +  or - , the modifiers in the list are added to or subtracted from the current modifier list. 

The following examples show how this works for the d  flag. An empty d  list enabled output for all debug macros. A nonempty list enables output only for the macro keywords in the list. 

These statements set the d  value to the modifier list as given: 

mysql> SET debug = ’d’; mysql> SELECT @@debug; +---------+ | @@debug | +---------+ | d | +---------+ mysql> SET debug = ’d,error,warning’; mysql> SELECT @@debug; +-----------------+ | @@debug | +-----------------+ | d,error,warning | +-----------------+ 

A leading +  or -  adds to or subtracts from the current d  value: 

mysql> SET debug = ’+d,loop’; mysql> SELECT @@debug; 

