Question

我有这个SQL：

SELECT        MONTH(data) AS MES, cor, CASE month(data) 
                          WHEN 1 THEN 'Janeiro' WHEN 2 THEN 'Fevereiro' WHEN 3 THEN 'Março' WHEN 4 THEN 'Abril' WHEN 5 THEN 'Maio' WHEN 6 THEN 'Junho' WHEN 7 THEN 'Julho' WHEN
                          8 THEN 'Agosto' WHEN 9 THEN 'Setembro' WHEN 10 THEN 'Outubro' WHEN 11 THEN 'Novembro' WHEN 12 THEN 'Dezembro' END AS MESCOR, COUNT(*) 
                          AS Expr1, CASE cor WHEN 'AM' THEN '2' WHEN 'VD' THEN '1' WHEN 'VM' THEN '3' END AS Expr2 

FROM            TBINICIATIVAS_PREVISTAS
WHERE        (login ='xxxxxxx')
GROUP BY MONTH(data), cor

UNION 
SELECT '1', 'VD', 'Janeiro',0,'1' 

UNION
SELECT '1', 'AM', 'Janeiro',0,'2' 

UNION
SELECT '1', 'VM', 'Janeiro',0,'3'


ORDER BY expr2, mes

但我需要UNION是有条件的。类似的东西：

if (select count(*) .... where cond1...) = 0
UNION 
SELECT '1', 'VD', 'Janeiro',0,'1' 

if (select  count(*) ....where cond2...) = 0
UNION
SELECT '1', 'AM', 'Janeiro',0,'2'

if (select  count(*) ....where cond3...) = 0
UNION
SELECT '1', 'VM', 'Janeiro',0,'3'

我试过，但我总是遇到语法错误。

有可能吗？

Answer 1

将您的条件移至WHERE UNION SELECT内的SELECT条款（UNION之间除SELECT '1', 'VD', 'Janeiro',0,'0' UNION SELECT '1', 'VD', 'Janeiro',0,'1' WHERE (select count(*) ....where cond1...) = 0 UNION SELECT '1', 'AM', 'Janeiro',0,'2' WHERE (select count(*) ....where cond2...) = 0 UNION SELECT '1', 'VM', 'Janeiro',0,'3' WHERE (select count(*) ....where cond3...) = 0之外的其他任何内容

count = 0

此外，我从您的条件中注意到您正在检查该行是否存在，方法是执行NOT EXISTS。用实际的SELECT '1', 'VD', 'Janeiro',0,'0' UNION SELECT '1', 'VD', 'Janeiro',0,'1' WHERE NOT EXISTS (select 1 ....where cond1...) UNION SELECT '1', 'AM', 'Janeiro',0,'2' WHERE NOT EXISTS (select 1 ....where cond2...) UNION SELECT '1', 'VM', 'Janeiro',0,'3' WHERE NOT EXISTS (select 1 ....where cond3...)替换它会更快，因为SQL引擎不必实际计算要与0进行比较的所有行，它会在找到行时立即返回。

template <typename T, 
          std::enable_if_t<std::is_trivially_copyable<T>::value, int> = 0>
inline int test_sfinae(T tc) {
    return 1;
}

template <typename T, 
          std::enable_if_t<!std::is_trivially_copyable<T>::value, int> = 0>
inline int test_sfinae(T ntc) {
    return 2;
}

条件UNION中的错误

1 个答案: