我可以在 MySQL SELECT 中执行准备好的语句吗？

Question

我在 MySQL 中运行搜索查询以从产品和价目表表中返回项目。当用户进行搜索查询时，我需要从产品表中获取产品 LEFT JOINED 与来自第二个表（任意数量的卖家）的所有授权/解锁卖家。

我有基本的搜索查询工作，但我无法让“动态左联接”工作。我在准备声明中被告知要这样做，我（a）正在努力（第一次）并且我不知道在工作时把它放在哪里。

我的搜索查询将如下所示：

SELECT articles AS art 
<< need to left join here >>
FROM bigtable AS bt
WHERE
  a lot of other criteria

这是我想出的准备声明：

SET @sql_text := '
    DECLARE strCount   INT DEFAULT 1;

    SELECT sid, ifnull(pricelist,"BASE"), count(*) AS recs
    FROM buyerList AS b
    LEFT JOIN sellerList AS s ON s.sid = b.sid AND s.pass = b.pass
    WHERE b.bid = ?

    SET @string = "LEFT JOIN preislisten AS p";

    lj:
      LOOP

        SET @string = CONCAT( @string, "ON (p.iln = a.iln AND p.preisliste = sid AND p.ean = a.ean AND p.iln = pricelist ) OR");

        SET strCount = strCount+1;
        IF strCount = recs
        THEN LEAVE lj;
        END IF;

    END LOOP lj;

    SET @string = CONCAT( @string,"(1=0)")
    ';

SET @param_iln = param_iln;
PREPARE stmt FROM @sql_text;
EXECUTE stmt using @param_iln;
DEALLOCATE PREPARE stmt;

所以我基本上是找到所有卖家和价格表名称（默认 BASE），然后尝试构造一个这样的字符串：

LEFT JOIN pricelists p ON
(p.sid = a.sid AND p.pricelist = "foo" AND p.ean = a.ean AND p.iln = 23467 ) OR 
(p.sid = a.sid AND p.pricelist = "bar" AND p.ean = a.ean AND p.iln = 99999) OR
... 
(1=0)

我的问题：
(1) 如果有 25 个卖家，我会做 25 个循环，但是正确的卖家和价目表会像这样插入到我的字符串中吗？
(2) 如何将其插入到我的实际搜索查询中？如果我执行，我可能会得到一个字符串，但我不能只将字符串放入查询中，对吗？

感谢您对我的宽容...第一次准备声明，第 2 周 MySQL...

编辑：
所以这就是我想出的：

# bigtable query
SELECT count(a.id) AS gesamt_datensaetze, a.nos, a.nos_anzeige
    FROM artikelstammdaten AS a

    #JOIN colors
    LEFT JOIN farbenzuordnung AS zu 
        ON a.farbe = zu.farbe 
        AND ( ( param_filter = '' ) AND (1=1) OR ( zu.ILN = param_filter ) )

  # ========== NEW PART ==========
  LEFT JOIN preislisten AS p
    ON (        p.iln = a.iln 
            AND p.EAN = a.EAN
            AND p.preisliste = (  #NEW GET pricelists      
                                                        SELECT IFNULL(klhs.preisliste, "-Standard-") AS pricelistID
                    FROM kundenliste_haendler AS klhd

                    LEFT JOIN kundenliste_hersteller AS klhs
                        ON klhs.iln = klhd.iln_verkaeufer
                        AND klhs.plz = klhd.plz
                        AND klhs.cid= klhd.cid
                                                WHERE klhd.iln_kaeufer = param_iln
                    GROUP BY pricelistID
                              )
           AND p.iln = ( #GET seller 
                  SELECT klhd.iln_verkaeufer AS sellerID
          FROM kundenliste_haendler AS klhd

                   LEFT JOIN kundenliste_hersteller AS klhs
                     ON klhs.iln = klhd.iln_verkaeufer
                     AND klhs.plz = klhd.plz
                     AND klhs.cid= klhd.cid

                     WHERE klhd.iln_kaeufer = param_iln
                     GROUP BY pricelistID
                     )
    )
    # active
    WHERE   a.aktiv = "ja"
    # more criteria

仍然试图找出这是否正常工作。如果有人可以对我是否可以将我的（相同的）嵌套选择合并为一个嵌套选择有一些见解，将不胜感激！

Answer 1

从问题来看，您编写的内容与 Oracle 所谓的匿名代码块类似。MySQL 不支持这种机制。

至于你想完成什么，你不需要存储过程。

尝试像这样组装 SQL 语句：

@stmt = 'SELECT sid, ifnull(pricelist,"BASE"), count(*) AS recs FROM buyerList AS b ';
@stmt = CONCAT(@stmt,'LEFT JOIN sellerList AS s ON s.sid = b.sid AND s.pass = b.pass ');
@stmt = CONCAT(@stmt,'LEFT JOIN pricelists p ON ');
SELECT GROUP_CONCAT(CONCAT('(p.sid = a.sid AND p.preisliste = ',sid,' AND p.ean = a.ean AND p.iln = ',pricelist',)') SEPARATOR ' OR ')
INTO @LeftJoinClause FROM preislisten;
@stmt = CONCAT(@stmt,@LeftJoinClause,' WHERE b.bid = ?');
SELECT @stmt\G

这将打印出所需的查询

如果它是所需的查询，则执行它

PREPARE sql FROM @stmt;
EXECUTE sql using @param_iln;
DEALLOCATE PREPARE sql;

试一试 ！！！

如果您确实希望查询很小而不对每个值进行硬编码，只需使用更直接的 LEFT JOIN 设置编写代码

SELECT p.sid, ifnull(p.pricelist,"BASE"), count(*) AS recs FROM buyerList AS b
LEFT JOIN sellerList AS s ON s.sid = b.sid AND s.pass = b.pass
LEFT JOIN pricelists p ON
    (p.sid = a.sid AND p.preisliste = a.sid AND p.ean = a.ean)
WHERE b.bid = ?
GROUP BY p.sid,p.pricelist;

我也刚刚注意到你有 a.sid 和 a.ean，哪个表的别名是a？

Answer 2

关注在 SELECTS 内运行存储过程的技术问题，不。

我创建了一个测试来证明内联 SQL 和存储过程的上下文是不同的：

mysql> delimiter $$
mysql> CREATE PROCEDURE test123 ()
    -> begin 
    -> SELECT NOW();
    -> end;
    -> $$
Query OK, 0 rows affected (1.04 sec)
mysql> delimiter ;
mysql> call test123();
+---------------------+
| NOW()               |
+---------------------+
| 2012-06-21 08:58:15 |
+---------------------+
1 row in set (0.04 sec)

Query OK, 0 rows affected (0.04 sec)

mysql> SELECT t.* FROM ( call test123() ) t;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'call test123() ) t' at line 1
mysql> SELECT ( call test123() ) AS t;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'call test123() ) AS t' at line 1