sql - SQL 中的 if-else 子句

Question

有没有办法在没有 PL/SQL 的情况下为此类请求创建查询：

if (select count(column) from table = 0)
then select column1, column2, column 3 from table where condition1
else select column1, column2, column 3 from table where condition2

Answer 1

select column1, column2, column3 
  from table 
 where (condition1 and (select count(column) from table) = 0)
    or (condition2 and (select count(column) from table) != 0)

Answer 2

SQL = SELECT QUERY;

if(Column != 0)
{
     SELECT QUERY;
}
if(Column2 != 0)
{
     SELECT QUERY;
}

当您不添加 else 语句时，它应该可以工作。

Answer 3

您可以使用两个 LEFT JOINS 和 COALESCE（在这里，我使用 column1 > 2 / column1 <=2 作为条件 1 和条件 2）：

with 
  v1 as (select count(column4) as c4_cnt from table1),
  v_cond1 as (select column1, column2, column3 from table1 where column1 > 2),
  v_cond2 as (select column1, column2, column3 from table1 where column1 <= 2)
select 
  v1.*, 
  coalesce(v_cond1.column1, v_cond2.column1) as column1,  
  coalesce(v_cond1.column2, v_cond2.column2) as column2,
  coalesce(v_cond1.column3, v_cond2.column3) as column3
from v1
left join v_cond1 on v1.c4_cnt = 0
left join v_cond2 on v1.c4_cnt != 0