如果存在数据，则...使用Oracle SQL?

问题描述：

此刻我有两个查询，返回相同的数据，如果第一个查询没有返回结果，我希望第二个查询运行，这是我现在的PHP的基本思想:

I have two queries at the moment, returning the same data, I want the second query to run if the first returns no results, here's the basic idea of my PHP right now:

Run Query1
If Query1 returns a result
    Enter data into array
Else 
    Run Query2
    Enter data into array

我想知道的是，我可以仅使用Oracle SQL来做到这一点吗?因此，它只对数据库运行一个查询，对第一条语句进行快速检查，如果没有找到结果，则运行第二条语句?

What I'm wondering, is can I do that exclusively using Oracle SQL? So it'd just run a single query to the database, do a quick check of the first statement, and if no results are found run the second one?

谢谢

答

QUERY1
UNION ALL
QUERY2
WHERE NOT EXISTS (QUERY1)

例如:

SELECT id
  FROM products
    UNION ALL 
SELECT  id
  FROM old_products
    WHERE NOT EXISTS (SELECT id  FROM products)

查询:

SELECT * 
    FROM Table 
WHERE ID = 7 
    AND Flag = 1
UNION ALL
SELECT * 
    FROM Table 
WHERE ID = 7 
    AND NOT EXISTS (select * from Table where ID = 7 and Flag = 1)

要确定唯一行，您也可以尝试以下操作:

To identifiy unique rows you can try this as well:

WITH qry AS
(
    SELECT a.* ,
        ROW_NUMBER() OVER(PARTITION BY id ORDER BY  Flag DESC ) rn
        FROM Table a
    WHERE ID = 7  -- THIS CAN BE PARAMETERIZED
)
SELECT * 
    FROM qry
WHERE rn = 1

如果存在数据，则...使用Oracle SQL?

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