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INCR/DECR Functions

Description

The INCR function increases the column's value given as a parameter of the SELECT statement by 1. The DECR function decreases the value of the column by 1.

Syntax

SELECT [ qualifier ] select_expression

[ { TO | INTO } variable [ {, variable }...; ] ]

...;

select_expression:

 *

 table_name. *

 [expression | counter_expression] [ {, expression |

counter_expression}...]

 

counter_expression :

INCR(path_expression)

The INCR and DECR functions are called "click counters" and can be effectively used to increase the number of post views for a Bulletin Board System (BBS) type of web service. In a scenario where you want to SELECT a post and immediately increase the number of views by 1 using an UPDATE statement, you can view the post and increment the number at once by using the INCR function in a single SELECT statement.

The INCR function increments the column value specified as an argument. Only integer type numbers can be used as arguments. If the value is NULL, the INCR function returns the NULL. That is, a value must be valid in order to be incremented by the INCR function. The DECR function decrements the column value specified as a parameter.

If an INCR function is specified in the SELECT statement, the COUNTER value is incremented by 1 and the query result is displayed with the values before the increment. Furthermore, the INCR function does not increment the value of the row(tuple) affected by the query process but rather the one affected by the final result.

Remark
  • The INCR/DECR functions execute independent of user-defined transactions and is applied automatically to the database by the top operation internally used in the system, apart from the transaction's COMMIT/ROLLBACK.
  • When multiple INCR/DECR functions are specified in a single SELECT statement, the failure of any of the INCR/DECR functions leads to the failure of all of them.
  • The INCR/DECR functions apply only to top-level SELECT statements. SUB SELECT statements such as INSERT ... SELECT ... statement and UPDATE table SET col = SELECT ... statement are not supported. The following example shows where the INCR function is not allowed.
  • SELECT b.content, INCR(b.read_count) FROM (SELECT * FROM board WHERE id = 1) AS b
  • If the SELECT statement with INCR/DECR functions returns more than one row as a result, it is treated as an error. The final result where only one row exists is valid.
  • The INCR/DECR function can be used only in numerical type. Applicable domains are limited to integer data types such as SMALLINT and INTEGER. They cannot be used in other types.
  • When the INCR function is called, the value to be returned will be the current value, while the value to be stored will be the current value + 1. Execute the following statement to select the value to be stored as a result :
  • SELECT content, INCR(read_count) + 1 FROM board WHERE id = 1;
  • If the defined maximum value of the type is exceeded, the INCR function initializes the column value to 0. Likewise, the column value is also initialized to 0 when the DECR function applies to the minimum value. 
  • Data inconsistency can occur because the INCR/DECR functions are executed regardless of UPDATE trigger. The following example shows the database inconsistency in that situation.
  • CREATE TRIGGER event_tr BEFORE UPDATE ON event EXECUTE REJECT;
  • SELECT INCR(players) FROM event WHERE gender='M';
  • The INCR/DECR functions returns an error in the write-protected broker mode such as slave mode of HA configuration, CSQL Interpreter (csql -r) of read-only, Read Only, Slave Only or Preferred Host Read Only mode.
Example

Suppose that the following three rows of data are inserted into the 'board' table.

CREATE TABLE board (

id  INT, title  VARCHAR(100), content  VARCHAR(4000), read_count  INT );

INSERT INTO board VALUES (1, 'aaa', 'text...', 0);

INSERT INTO board VALUES (2, 'bbb', 'text...', 0);

INSERT INTO board VALUES (3, 'ccc', 'text...', 0);

The following example shows how to increment the value of the 'read_count' column in data whose 'id' value is 1 by using the INCR function.

SELECT content, INCR(read_count) FROM board WHERE id = 1;

  content                read_count

===================================

  'text...'                       0

In the example, the column value becomes read_count + 1 as a result of the INCR function in the SELECT statement. You can check the result using the following SELECT statement.

SELECT content, read_count FROM board WHERE id = 1;

  content                read_count

===================================

  'text...'                       1