Question

我已经测试了这个查询（见下文）并且它无效

我有这张表：

id     A      B      C
-----------------------
1      1      2     
2             3     
3      2             4
4      6      2      1
-----------------------

我想在第一个空字段中将A，B或C设置为4，除非4已经在A，B或C中。

因此，如果（条件1） A！= 4且B！= 4且C！= 4 =＆gt; （条件2）如果A！=“”转到B，如果B！=“”转到C，如果C！=“”跳过更新。

In my example 
row 1: C => 4
row 2: A => 4
row 3: not updated (4 allready exists)
row 4: not updated (no empty field)

我提出的查询：

UPDATE table SET 
    A=IF(A!="",A,4),                 //1:
    B=IF(A="" OR B!="",B,4),         //2:
    C=IF(A="" OR B="",C,4)           //3:
WHERE (A!="" OR B!="" OR C!="")      //4:
       AND (A!=4 AND B!=4 AND C!=4)  //5: 

       // 1: if A="", make it 4, else keep value  
       // 2: if A was "" it should be 4 by now, let B be B, else make B 4
       // 3: if A was "" or B was "", one of them should be 4 by now, else make C 4
       // 4: eliminates row 4 (check if there's a free field)
       // 5: eliminates row 3 (condition 1)

这个查询是否有效（特别是第2部分和第3部分）取决于mySQL的作用：它会查看IF语句中字段的 OLD 值，还是首先更新一个字段和然后将新值带到下一部分？测试朗姆酒失败了，它将所有空字段更新为4，表明它考虑了新值。

所以我的问题是：我该怎么做才能使查询符合我的条件？

[编辑]

不是最优雅的查询，但这有效：

UPDATE table SET 
    A=IF( A<>"",A,4),                
    B=IF( (A="" OR A=4) OR B<>"",B,4),        
    C=IF( (A="" OR A=4) OR (B="" OR B=4) OR C<>"",C,4)         
WHERE (A!="" OR B!="" OR C!="")     
       AND (A!=4 AND B!=4 AND C!=4)

似乎mySQL将A的新值带到B的IF语句中，并将A和B的新值带到C

Answer 1

试

UPDATE table SET 
    A=IF(A is null,4,A),   -- if a is null, set it to 4            
    B=IF(A is not null and b is null,4,B),       -- if A did not meet the condition to set to 4 but B is null, set B to 4
    C=IF(A is not null and B is not null and C is null ,4,C) --  if A or B did not meet the condition to be set to 4 and C is null, set c to 4
WHERE (a+b+c is null)     --- short of "a is null or b is null or c is null"
       AND (A!=4 AND B!=4 AND C!=4)

根据旧值更新多个字段

1 个答案: