Question

我有一张桌子

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

如何创建一个唯一索引，以确保该表只能有一行其中belongssto_userid为null？

这不起作用：

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

Answer 1

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

So what it does - it creates a partial index over expression: that has explicitly the fixed value 1 for every row where belongsto_userid IS NULL.

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.

So extra parentheses are required by the syntax.

References:

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

Answer 2

The code to create index needs column name:

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

唯一的空索引

2 个答案: