Question

select x+y as z,  
case  
    when "x"+"y"  < 0 then "Less Than Zero"  
    when "x"+"y"  > 0 then "Non Zero"  
    else "Zero"  
end  
from sometable;

返回预期结果，但多次对每行数据进行添加。

我正在尝试优化查询，但不能正常工作..

select x+y as z,  
case  
    when "z" < 0 then "Less Than Zero"  
    when "z" > 0 then "Non Zero"  
    else "Zero"  
end  
from sometable;

始终返回＆＃34;小于零＆＃34;。

我在这个查询上做错了什么？如何在执行查询时多次避免添加A和B？

Answer 1

SELECT子句中的列别名在同一SELECT子句中的其他表达式中不可用。（SELECT x AS y, y AS x ...应该怎么办？）

您可以通过将其移动到子查询来使这样的别名可用：

SELECT z,
       CASE WHEN z < 0 THEN 'Less Than Zero'
            WHEN z > 0 THEN 'Non Zero'
            ELSE            'Zero'
       END
FROM (SELECT x + y AS z
      FROM sometable);

然而，这只能节省打字;它实际上并没有优化重复计算：

sqlite> explain select z, z from (select x+y as z from sometable);
addr  opcode         p1    p2    p3    p4             p5  comment
----  -------------  ----  ----  ----  -------------  --  -------------
0     Init           0     11    0                    00  Start at 11
1     OpenRead       1     2     0     2              00  root=2 iDb=0; sometable
2     Rewind         1     9     0                    00
3       Column         1     0     3                    00  r[3]=sometable.x
4       Column         1     1     4                    00  r[4]=sometable.y
5       Add            4     3     1                    00  r[1]=r[4]+r[3]
6       Add            4     3     2                    00  r[2]=r[4]+r[3]
7       ResultRow      1     2     0                    00  output=r[1..2]
8     Next           1     3     0                    01
9     Close          1     0     0                    00
10    Halt           0     0     0                    00
11    Transaction    0     0     1     0              01  usesStmtJournal=0
12    TableLock      0     2     0     sometable      00  iDb=0 root=2 write=0
13    Goto           0     1     0                    00

如何优化常见的子表达式？

1 个答案: