Question

class LegacyHashServiceProvider extends ServiceProvider
{
    protected $defer = true;

    public function register()
    {
        $this->app->singleton('hash', \App\Classes\LegacyHash::class);
    }

    public function provides()
    {
        return ['hash'];
    }
}

我想用条件更新列。

在第一行，

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

在第二行，

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

同样适用于第三行

Answer 1

我认为你可以用case语句和变量做你想做的事情：

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;

SQL使用条件更新多行

1 个答案: