Question

Trying to figure out why it is NULL. I was expecting 7 to be printed.

mysql> set @total = 0;
Query OK, 0 rows affected (0.00 sec)

mysql> call getAuthorCount(@total);
+------------------------+
| count(distinct author) |
+------------------------+
|                      7 |
+------------------------+
1 row in set (0.00 sec)

Query OK, 0 rows affected (0.02 sec)

mysql> select @total as totalauthors;
+--------------+
| totalauthors |
+--------------+
|         NULL |
+--------------+

The procedure,

mysql> create procedure getAuthorCount(out authorcount int)
    -> begin
    ->  select count(distinct author) from libbooks;
    -> end
    -> //

Answer 1

You should use INOUT parameter –

CREATE PROCEDURE getAuthorCount(INOUT authorcount INT)
BEGIN
  SELECT count(DISTINCT author) FROM libbooks;
END

---

Examples:

When @total value is as is (0 in, 0 out):

DROP PROCEDURE getAuthorCount;
DELIMITER $$
CREATE PROCEDURE getAuthorCount(INOUT authorcount INT)
BEGIN
  -- SET authorcount = 100;
END$$
DELIMITER ;

SET @total = 0;
CALL getAuthorCount(@total);
SELECT @total AS totalauthors;
+--------------+
| totalauthors |
+--------------+
|            0 |
+--------------+

When @total value is replaces with a new value in stored procedure:

DROP PROCEDURE getAuthorCount;
DELIMITER $$
CREATE PROCEDURE getAuthorCount(OUT authorcount INT)
BEGIN
  SET authorcount = 100;
END$$
DELIMITER ;

SET @total = 0;
CALL getAuthorCount(@total);
SELECT @total AS totalauthors;
+--------------+
| totalauthors |
+--------------+
|          100 |
+--------------+