13.6.7.3 GET DIAGNOSTICS Syntax

SQL statements produce diagnostic information that populates the diagnostics area. The GET DIAGNOSTICS statement enables applications to inspect this information. (You can also use SHOW WARNINGS or SHOW ERRORS to see conditions or errors.)

No special privileges are required to execute GET DIAGNOSTICS.

The keyword CURRENT means to retrieve information from the current diagnostics area. The keyword STACKED means to retrieve information from the second diagnostics area, which is available only if the current context is a condition handler. If neither keyword is given, the default is to use the current diagnostics area.

The GET DIAGNOSTICS statement is typically used in a handler within a stored program. It is a MySQL extension that GET [CURRENT] DIAGNOSTICS is permitted outside handler context to check the execution of any SQL statement. For example, if you invoke the mysql client program, you can enter these statements at the prompt:

This extension applies only to the current diagnostics area. It does not apply to the second diagnostics area because GET STACKED DIAGNOSTICS is permitted only if the current context is a condition handler. If that is not the case, a GET STACKED DIAGNOSTICS when handler not active error occurs.

For a description of the diagnostics area, see Section 13.6.7.7, “The MySQL Diagnostics Area”. Briefly, it contains two kinds of information:

For a statement that produces three conditions, the diagnostics area contains statement and condition information like this:

Statement information:
  row count
  ... other statement information items ...
Condition area list:
  Condition area 1:
    error code for condition 1
    error message for condition 1
    ... other condition information items ...
  Condition area 2:
    error code for condition 2:
    error message for condition 2
    ... other condition information items ...
  Condition area 3:
    error code for condition 3
    error message for condition 3
    ... other condition information items ...

GET DIAGNOSTICS can obtain either statement or condition information, but not both in the same statement:

The retrieval list specifies one or more target = item_name assignments, separated by commas. Each assignment names a target variable and either a statement_information_item_name or condition_information_item_name designator, depending on whether the statement retrieves statement or condition information.

Valid target designators for storing item information can be stored procedure or function parameters, stored program local variables declared with DECLARE, or user-defined variables.

Valid condition_number designators can be stored procedure or function parameters, stored program local variables declared with DECLARE, user-defined variables, system variables, or literals. A character literal may include a _charset introducer. A warning occurs if the condition number is not in the range from 1 to the number of condition areas that have information. In this case, the warning is added to the diagnostics area without clearing it.

When a condition occurs, MySQL does not populate all condition items recognized by GET DIAGNOSTICS. For example:

In standard SQL, if there are multiple conditions, the first condition relates to the SQLSTATE value returned for the previous SQL statement. In MySQL, this is not guaranteed. To get the main error, you cannot do this:

Instead, retrieve the condition count first, then use it to specify which condition number to inspect:

For information about permissible statement and condition information items, and which ones are populated when a condition occurs, see Diagnostics Area Information Items.

Here is an example that uses GET DIAGNOSTICS and an exception handler in stored procedure context to assess the outcome of an insert operation. If the insert was successful, the procedure uses GET DIAGNOSTICS to get the rows-affected count. This shows that you can use GET DIAGNOSTICS multiple times to retrieve information about a statement as long as the current diagnostics area has not been cleared.

Suppose that t1.int_col is an integer column that is declared as NOT NULL. The procedure produces these results when invoked to insert non-NULL and NULL values, respectively:

When a condition handler activates, a push to the diagnostics area stack occurs:

The next example shows how GET STACKED DIAGNOSTICS can be used within a handler to obtain information about the handled exception, even after the current diagnostics area has been modified by handler statements.

Within a stored procedure p(), we attempt to insert two values into a table that contains a TEXT NOT NULL column. The first value is a non-NULL string and the second is NULL. The column prohibits NULL values, so the first insert succeeds but the second causes an exception. The procedure includes an exception handler that maps attempts to insert NULL into inserts of the empty string:

When the handler activates, a copy of the current diagnostics area is pushed to the diagnostics area stack. The handler first displays the contents of the current and stacked diagnostics areas, which are both the same initially:

+---------------------------------+-------+----------------------------+
| op                              | errno | msg                        |
+---------------------------------+-------+----------------------------+
| current DA before mapped insert |  1048 | Column 'c1' cannot be null |
+---------------------------------+-------+----------------------------+

+---------------------------------+-------+----------------------------+
| op                              | errno | msg                        |
+---------------------------------+-------+----------------------------+
| stacked DA before mapped insert |  1048 | Column 'c1' cannot be null |
+---------------------------------+-------+----------------------------+

Statements executing after the GET DIAGNOSTICS statements may reset the current diagnostics area. statements may reset the current diagnostics area. For example, the handler maps the NULL insert to an empty-string insert and displays the result. The new insert succeeds and clears the current diagnostics area, but the stacked diagnostics area remains unchanged and still contains information about the condition that activated the handler:

+----------------------------------------------+
| op                                           |
+----------------------------------------------+
| mapped insert succeeded, current DA is empty |
+----------------------------------------------+

+--------------------------------+-------+----------------------------+
| op                             | errno | msg                        |
+--------------------------------+-------+----------------------------+
| stacked DA after mapped insert |  1048 | Column 'c1' cannot be null |
+--------------------------------+-------+----------------------------+

When the condition handler ends, its current diagnostics area is popped from the stack and the stacked diagnostics area becomes the current diagnostics area in the stored procedure.

After the procedure returns, the table contains two rows. The empty row results from the attempt to insert NULL that was mapped to an empty-string insert:

+----------+
| c1       |
+----------+
| string 1 |
|          |
+----------+

In the preceding example, the first two GET DIAGNOSTICS statements within the condition handler that retrieve information from the current and stacked diagnostics areas return the same values. This will not be the case if statements that reset the current diagnostics area execute earlier within the handler. Suppose that p() is rewritten to place the DECLARE statements within the handler definition rather than preceding it:

In this case, the result is version dependent:

To avoid this issue within a condition handler when seeking to obtain information about the condition that activated the handler, be sure to access the stacked diagnostics area, not the current diagnostics area.