合并两个 SQL 查询

Question

我需要一个模仿 if-then-else 语句形式的 SQL 查询：

if (query1 != null)
  return query1
else
  return query2

由于 COALESCE 不适用于结果集，我创建了一个联合查询来完成这项工作：

SELECT * FROM obs WHERE cond1  --query1
UNION
SELECT * FROM obs WHERE (NOT EXISTS(query1)) AND cond2

在 SQL 中：

  ( SELECT * FROM obs WHERE src = @id AND tstart <= @instant AND tend >= @instant )
  UNION
  ( SELECT * FROM obs WHERE (NOT EXISTS (SELECT 1 FROM obs WHERE src = @id AND tstart <= @instant AND tend >= @instant )) AND src = @id AND tstart <= @instant ORDER BY tend DESC LIMIT 1);

表 obs 具有字段 ( src | tstart |ten | ... )。我想选择那些与@instant 重叠的行。如果没有找到重叠行，则应返回@instant 之前最近的行。

SQL UNION 语句有效，但非常笨拙，我正在寻找更短更清晰的语句。符合 COALESCE ( query1, query2 ) 精神的东西会很好。我的数据库是 Postgresql。

Answer 1

这对我有用。我合并了这两个查询，然后对记录进行了排名。它将在 table2 之前从 table1 中获取记录。

with prev_tables as (
        select name, id, address, 1 as ranking
        from table1
        union 
        select name, id, address, 2
        from table2
    ),
    prev_dedupe as (
        select p.*, rank() over (partition by id order by ranking) as prev_table_rk
        from prev_tables p
    )
select * from prev_dedupe where prev_table_rk = 1

Answer 2

首先，在这种情况下，union all 可能比 union 更合适。

其次，您可以使用with 来表达这一点，以简化查询：

WITH t1 as (
      SELECT *
      FROM obs
      WHERE src = @id AND tstart <= @instant AND tend >= @instant
     )
SELECT t1.*
FROM t1
UNION ALL
(SELECT *
 FROM obs
 WHERE NOT EXISTS (SELECT 1 FROM t1) AND
       src = @id AND tstart <= @instant
 ORDER BY tend DESC
 LIMIT 1
);

但是，如果您正在寻找单行，这更简单：

 SELECT *
 FROM obs
 WHERE src = @id
 ORDER BY (CASE WHEN  tstart <= @instant AND tend >= @instant THEN 1
                ELSE 2
           END),
          tend DESC
 LIMIT 1;

而且，如果不是单行，那么也可以使用窗口函数：

SELECT o.*
FROM (SELECT o.*,
             DENSE_RANK() OVER (PARTITION BY src
                                ORDER BY (CASE WHEN tstart <= @instant AND tend >= @instant THEN 1
                                               ELSE 2
                                          END),
                                         (CASE WHEN tstart <= @instant AND tend >= @instant THEN NULL
                                               ELSE tend
                                          END) DESC
                               ) as seqnum
      FROM obs o
      WHERE src = @id
     ) o
WHERE seqnum = 1;