问题描述
如果我有一个表,我想根据插入期间可以为空的多个列来强制执行唯一性(使用 postgres),我是否需要创建多个唯一索引,如下所示?
CREATE TABLE IF NOT EXISTS table_a (
id SERIAL PRIMARY KEY,table_b_id INT REFERENCES table_b(id) ON DELETE CASCADE,column_a INT,column_b INT,column_c INT,created_time BIGINT NOT NULL,CONSTRAINT "table_a_key" UNIQUE ("table_b_id","column_a","column_b","column_c")
);
column_a
、column_b
和 column_c
都可以为 null。我只想要表中每条记录的这些列的唯一组合。
有效插入
table_b_id | column_a | column_b | column_c
1 | 1 | 2 | 3
1 | 1 | null | null
1 | null | 2 | 3
1 | null | 2 | null
但是,如果我再次尝试插入 1 | null | 2 | null
,它会发生冲突并且不会插入。
认为仅添加 CONSTRAINT "table_a_key" UNIQUE...
仅在所有列都没有任何空值的情况下强制唯一性。
我是否需要创建所有这些索引来强制唯一性?有没有更好的办法?
CREATE UNIQUE INDEX table_all_null_index ON table_a (table_b_id)
WHERE column_a IS NULL AND column_b IS NULL AND column_c IS NULL;
CREATE UNIQUE INDEX table_a_col_a_index ON table_a (table_b_id,column_a)
WHERE column_a IS NOT NULL AND column_b IS NULL AND column_c IS NULL;
CREATE UNIQUE INDEX table_a_col_b_index ON table_a (table_b_id,column_b)
WHERE column_b IS NOT NULL AND column_a IS NULL AND column_c IS NULL;
CREATE UNIQUE INDEX table_a_col_c_index ON table_a (table_b_id,column_c)
WHERE column_c IS NOT NULL AND column_a IS NULL AND column_b IS NULL;
CREATE UNIQUE INDEX table_a_col_a_b_index ON table_a (table_b_id,column_a,column_b)
WHERE column_a IS NOT NULL AND column_b IS NOT NULL AND column_c IS NULL;
CREATE UNIQUE INDEX table_a_col_a_c_index ON table_a (table_b_id,column_c)
WHERE column_a IS NOT NULL AND column_c IS NOT NULL AND column_b IS NULL;
CREATE UNIQUE INDEX table_a_col_b_c_index ON table_a (table_b_id,column_b,column_c)
WHERE column_b IS NOT NULL AND column_c IS NOT NULL AND column_a IS NULL;
解决方法
如果您想允许多个 NULL
列,那么这就是常规唯一索引或约束的作用:
unique (a,b,c)
如果不想让 NULL
值重复,那么最简单的方法就是将它们默认为一个通用值,例如 -1
:
unique (coalesce(a,-1),coalesce(b,coalesce(c,-1))
这可能并非在所有情况下都有效,但在许多情况下都有效,因为 -1
(或其他一些值)并未以其他方式使用。