SQL更新條件

Question

Name id Col1 Col2 Col3 
Row1 1 6  1  A  
Row2 2 2  3  B  
Row3 3 9  5  B  
Row4 4 16 8  C  

多行我想更新與狀態欄。

在第一行，

update col2 = Col1+0 if Col3 = A 
OR 
col2 = Col1-0 if Col3 = B 
OR 
col2 = Col1*0 if Col3 = C 

在第二排，

update col2 = (previous col2) + Col1 if Col3 = A 
OR 
col2 = (previous col2) - Col1 if Col3 = B 
OR 
col2 = (previous col2) * Col1 if Col3 = C 

同樣進入第三排

Answer 1

我認爲你可以做你想做的一個case語句和變量：

set @prev_col2 = 0; 

update t 
    set col2 = (case when (@col2 := @prev_col2) = NULL then -1 -- never happens 
        when (@prev_col2 := col2) = NULL then -1 -- never happens 
        when col3 = 'A' then Col1 + @col2 
        when col3 = 'B' then Col1 - @col2 
        when Col3 = 'C' then col1 * @col2 
       end) 
    order by id;