Question

假设f是一个字符串函数，例如串联0：f（0011）= 00110。

假设mytable列string是一个索引，price。

我想做类似下面的伪代码：

UPDATE mytable SET price(X)=0.5 FOR ALL THOSE ROWS X SUCH THAT
THE ROW Y SUCH THAT THE `string` OF Y equals f(the `string` FOR X)
HAS price(Y)=0

更一般地说，我该如何参考

＆＃34;这样的行[条件保持 - 取决于当前行]＆＃34;

in（My）SQL？

Answer 1

将UPDATE与自我加入一起使用。

UPDATE mytable AS x
JOIN mytable AS y ON y.string = f(x.string)
SET x.price = 0.5
WHERE y.price = 0

Answer 2

设置演示

CREATE TABLE mytable 
( id         INT UNSIGNED NOT NULL PRIMARY KEY
, thestring  VARCHAR(32)
, price      DECIMAL(11,2)
) ENGINE=INNODB
;
INSERT INTO mytable (id, thestring, price) VALUES
 ( 1,'0011'   , 123.45)
,( 2,'00110'  ,  45.67)
,( 3,'001100' ,   0.00)   
,( 4,'10'     ,   4.44)
,( 5,'100'    ,   5.55)
,( 6,'1000'   ,   0.00)
;

编写一个标识mytable

中Y行的查询

SELECT y.*
  FROM `mytable` `y` 
 WHERE y.price = 0.0 
 ORDER BY y.id

向mytable添加联接以查找匹配的X行

SELECT x.id         AS x_id
     , x.thestring  AS x_thestring 
     , x.price      AS x_price
     , y.id         AS y_id 
     , y.thestring  AS y_thestring
     , y.price      AS y_price
  FROM mytable `y`
  JOIN mytable `x` 
    ON CONCAT(x.thestring,'0') = y.thestring 
 WHERE y.price = 0.0 
 ORDER BY y.id

将SELECT转换为UPDATE。（将SELECT ... FROM替换为UPDATE，并在WHERE子句之前添加SET子句。

UPDATE mytable `y`
  JOIN mytable `x` 
    ON CONCAT(x.thestring,'0') = y.thestring
   SET x.price = 0.5
 WHERE y.price = 0.0

只需将CONCAT(x.thestring,'0')替换为f(x.thestring)即可。

另一个选择是使用相关子查询。

首先，写一个SELECT

SELECT x.*
  FROM mytable `x`
 WHERE EXISTS ( SELECT 1
                  FROM mytable `y`
                 WHERE y.thestring = f(x.thestring)
                   AND y.price = 0.00
              )

然后将其转换为UPDATE。将SELECT ... FROM替换为UPDATE关键字，并在SET子句之前添加WHERE子句。

UPDATE mytable `x`
   SET x.price = 0.5
 WHERE EXISTS ( SELECT 1
                  FROM mytable `y`
                 WHERE y.thestring = f(x.thestring)
                   AND y.price = 0.00
              )

SQL Update：这样的行......？

2 个答案: