如果条件为真，则在MySQL存储过程中运行查询

问题描述

这花了一些工夫，但我做了足够的调整。您的代码问题与您的逻辑无关，但与MysqL存储过程语言本身无关。在执行动态sql时，存在范围问题。

我所做的是创建一个临时表并将返回的值存放在其中

这是一些示例数据

MysqL> drop database if exists user391986;
Query OK, 1 row affected (0.08 sec)

MysqL> create database user391986;
Query OK, 1 row affected (0.00 sec)

MysqL> use user391986
Database changed
MysqL> CREATE TABLE mytable (
    -> ID BIGINT NOT NULL AUTO_INCREMENT PRIMARY KEY,
    -> Name VARCHAR(255) NOT NULL,
    -> Value VARCHAR(255) NOT NULL
    -> ) ENGINE=InnoDB;
Query OK, 0 rows affected (0.11 sec)

MysqL> INSERT INTO mytable (Name,Value) VALUES
    -> ('rolando','edge'),('pamela','washington'),
    -> ('dominique','wilkins'),('diamond','cutter');
Query OK, 4 rows affected (0.06 sec)
Records: 4  Duplicates: 0  Warnings: 0

MysqL> SELECT * from mytable;
+----+-----------+------------+
| ID | Name      | Value      |
+----+-----------+------------+
|  1 | rolando   | edge       |
|  2 | pamela    | washington |
|  3 | dominique | wilkins    |
|  4 | diamond   | cutter     |
+----+-----------+------------+
4 rows in set (0.00 sec)

MysqL>

这是为适应临时表中的返回值而进行调整的存储过程

MysqL> delimiter //
MysqL> CREATE PROCEDURE myproc(IN myTable VARCHAR(255), IN myValue VARCHAR(255), IN myValueTwo VARCHAR(255))
    -> BEGIN
    ->     DECLARE foundcount INT;
    ->     DECLARE retval VARCHAR(255);
    ->
    ->     SET @iTable=myTable;
    ->     SET @iValue=myValue;
    ->     SET @iValueTwo=myValueTwo;
    ->
    ->     CREATE TEMPORARY TABLE IF NOT EXISTS mynumber (rv VARCHAR(255)) ENGINE=MEMORY;
    ->     DELETE FROM mynumber;
    ->
    ->     SET retval = 'nothing retrieved';
    ->     SET @query = CONCAT('INSERT INTO mynumber SELECT Name FROM ', @iTable, ' WHERE Value=''', @iValue, '''');
    ->     PREPARE QUERY FROM @query;
    ->     EXECUTE QUERY;
    ->     DEALLOCATE PREPARE QUERY;
    ->     SELECT COUNT(1) INTO foundcount FROM mynumber;
    ->     IF foundcount = 0 THEN
    ->         SET @querytwo = CONCAT('INSERT INTO mynumber SELECT Name FROM ', @iTable, ' WHERE Value=''', @iValueTwo, '''');
    ->         PREPARE QUERY FROM @querytwo;
    ->         EXECUTE QUERY;
    ->         DEALLOCATE PREPARE QUERY;
    ->     END IF;
    ->     SELECT COUNT(1) INTO foundcount FROM mynumber;
    ->     IF foundcount > 0 THEN
    ->         SELECT rv INTO retval FROM mynumber;
    ->     END IF;
    ->     SELECT retval;
    ->
    -> END //
Query OK, 0 rows affected (0.00 sec)

MysqL> delimiter ;
MysqL>

好的，我三次调用了存储过程。首先什么也没有。第二个获得第二个值。第三个获得第一个值。

MysqL> CALL myproc('mytable','pamela','diamond');
+-------------------+
| retval            |
+-------------------+
| nothing retrieved |
+-------------------+
1 row in set (0.00 sec)

Query OK, 0 rows affected (0.02 sec)

MysqL> CALL myproc('mytable','pamela','wilkins');
+-----------+
| retval    |
+-----------+
| dominique |
+-----------+
1 row in set (0.00 sec)

Query OK, 0 rows affected (0.01 sec)

MysqL> CALL myproc('mytable','edge','wilkins');
+---------+
| retval  |
+---------+
| rolando |
+---------+
1 row in set (0.00 sec)

Query OK, 0 rows affected (0.02 sec)

MysqL>

试试看！！！

解决方法

我正在使用MySQL数据库并尝试创建存储过程。我怎样才能做到，如果query1的结果没有记录，那么它将执行一个不同的查询？

这是我到目前为止的内容：

/* CREATE DB */
CREATE DATABASE mydata;
use mydata;
/* TABLE */
CREATE TABLE mydata (
ID BIGINT NOT NULL AUTO_INCREMENT PRIMARY KEY,Name                        VARCHAR(255) NOT NULL,Value                       VARCHAR(255) NOT NULL
) ENGINE=InnoDB;
INSERT INTO mydata (Name,Value) VALUES ("testname","testvalue");

/* STORED PROCEDURE */
delimiter //
CREATE PROCEDURE myproc(IN myTable VARCHAR(255),IN myValue VARCHAR(255),IN myValueTwo VARCHAR(255))
BEGIN
    SET @iTable=myTable;
    SET @iValue=myValue;
    SET @iValueTwo=myValueTwo;

    SET @query = CONCAT('SELECT Name FROM ',@iTable,' WHERE Value="',@iValue,'"');
    SET @querytwo = CONCAT('SELECT Name FROM ',@iValueTwo,'"');
    PREPARE QUERY FROM @query;
    EXECUTE QUERY;

END //
delimiter ;

/* CALL */
call myproc("mydata","testvalue","");

我想运行一个查询，并且仅在第一个没有行的情况下才执行第二个查询。做这个的最好方式是什么？

sql sql