我有一个像这样的 3 列的表
CREATE TABLE my_table (
A_id INTEGER NOT NULL,
B_id INTEGER NOT NULL,
C CHARACTER VARYING(50) NOT NULL,
PRIMARY KEY (A_id, B_id, C)
);
我必须用其他值更新 B_id 列中的某些值。 为此我尝试了这个查询
UPDATE my_table
SET B_id = 10
WHERE B_id = 20
但问题是这给了我一个“重复键值违反唯一约束”
因为在某些地方表有这样的数据
A_id, B_id, C
1, 10, a
1, 20, a ## this row seems to cause constraint problem
1, 20, b
2, 20, a
2, 20, b
我想让上面的数据变成这样
A_id, B_id, C
1, 10, a
1, 10, b
2, 10, a
2, 10, b
所以一般来说,如果组合键已经存在,则删除值为 20 的组合键,否则更新值。 感谢期待!
最佳答案
...如果组合键已经存在则忽略...
如果 ignore 表示:不更新,那么您可以使用 exists(tuple_with _new_values in the same table)
,如下所示:
UPDATE my_table mt
SET B_id = 10
WHERE mt.B_id = 20
AND NOT EXISTS ( SELECT *
FROM my_table nx -- same table
WHERE nx.A_id = mt.A_id -- same value
AND nx.B_id = 10 -- new value
AND nx.C_id = mt.C_id -- same value
);
[更新] 问题改变后。您可以使用 CTE 组合两个操作:
- 首先:删除更新会冲突的记录
- 第二:更新未删除的记录
WITH del AS ( -- delete tuples for which UPDATE would cause a conflict
DELETE FROM my_table mt
WHERE mt.B_id = 20
AND EXISTS ( SELECT *
FROM my_table nx
WHERE nx.A_id = mt.A_id
AND nx.B_id = 10
AND nx.C_id = mt.C_id
)
RETURNING *
)
UPDATE my_table upd -- UPDATE the records that were not deleted
SET B_id = 10
WHERE upd.B_id = 20
AND NOT EXISTS ( SELECT *
FROM del
WHERE del.A_id = upd.A_id
AND del.B_id = upd.B_id
AND del.C_id = upd.C_id
);
关于用于更新复合键中的列的 SQL 查询,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/44256902/