唯一空索引

Question

我有一个table

CREATE TABLE mandantconfigentity
(
  id bigint NOT NULL,
  currentlegalversionnumber integer NOT NULL DEFAULT 5,
  belongsto_userid character varying(255)
)

我如何创建唯一索引，以确保 table 只能有一行 belongsto_userid 为空？

这不起作用：

CREATE UNIQUE INDEX unique_mandantconfig_for_null_belongsto 
    ON mandantconfigentity (1) 
 WHERE (belongsto_userid IS NULL);

Answer 1

创建索引的代码需要列名：

CREATE UNIQUE INDEX unique_mandantconfig_for_null_belongsto 
    ON mandantconfigentity (id) 
WHERE (belongsto_userid IS NULL);

Answer 2

CREATE UNIQUE INDEX unique_mandantconfig_for_null_belongsto 
    ON mandantconfigentity ((1)) 
WHERE (belongsto_userid IS NULL);

那么它做了什么 - 它在表达式上创建了一个部分索引：对于 belongsto_userid IS NULL.

的每一行都有明确的固定值 1

The syntax of the CREATE INDEX command normally requires writing parentheses around index expressions, as shown in the second example. The parentheses can be omitted when the expression is just a function call, as in the first example.

因此语法需要额外的括号。

参考文献：

http://www.postgresql.org/docs/current/static/indexes-expressional.html

唯一空索引

Unique null index

sql

postgresql

unique-constraint