sql - 有没有更好的方法来调整这个查询？

Question

我有以下查询，想知道是否有更好的方法来调整它？

MERGE INTO target_table TARGET USING
(SELECT DISTINCT g1.column_name
FROM test_gtt1 g1,
test_gtt2 g2
WHERE g1.column_name = g2.column_name
AND g1.diff_id      IS NULL
AND g2.diff_id      IS NULL
UNION ALL
SELECT DISTINCT g1.column_name
FROM test_gtt1 g1,
test_gtt2 g2
WHERE 
g1.column_name  = g2.column_name
AND g1.diff_id = g2.diff_id
AND g1.diff_id IS NOT NULL
AND g2.diff_id IS NOT NULL
) SOURCE ON ( TARGET.column_name = SOURCE.column_name)
WHEN MATCHED THEN
UPDATE SET TARGET.column_name2 = 123;

Answer 1

这取决于数据类型、索引和生成的计划是什么（如果您提供了它会很有帮助）。

但是联合显然是多余的：

SELECT DISTINCT g1.column_name
FROM test_gtt1 g1,
test_gtt2 g2
WHERE g1.column_name = g2.column_name
AND ((g1.diff_id IS NULL
      AND g2.diff_id IS NULL)
     OR (g1.diff_id IS NOT NULL
         AND g2.diff_id IS NOT NULL))

更新

我突然想到（在回复 APC 的评论时）也可以使用...获得相同的结果。

SELECT DISTINCT g1.column_name
FROM test_gtt1 g1,
test_gtt2 g2
WHERE g1.column_name = g2.column_name
AND DECODE(g1.diff_id, NULL, -1, 1)=DECODE(g2.diff_id, NULL, -1, 1);

Answer 2

如果您要将这两组与 UNION 结合使用diff_id is null，则无需测试：diff_id is not null

       select * from foo where bar is null
       union all
       select * from foo where bar is not null

是相同的

       select * from foo