[英]Postgres constraint
postgres中有沒有一種創建約束的方法,如下所示:
我有一個值為“ time_of_day”的實體。 該值可以是早上,下午,晚上,白天,晚上或任何時間。
所以我試圖找出如何允許以下組合:
(2)已完成,因為它只是time_of_day的標准唯一約束。 我如何實現(1)。 可能嗎?
這是“容易的”,因為PostgreSQL如此可擴展。 您可以定義自己的類型,類型的比較運算符以及與btree
索引一起使用的運算符類,以便PostgreSQL知道如何比較它們。
訣竅是以沖突值相等的方式定義“相等”。
首先,我們定義類型:
CREATE TYPE tod AS ENUM ('morning', 'afternoon', 'anytime');
然后,我們定義索引支持例程,以便btree
索引知道如何比較值:
CREATE FUNCTION tod_compare(tod, tod) RETURNS integer
IMMUTABLE LANGUAGE sql AS
$$SELECT CASE WHEN $1 = 'morning' AND $2 = 'afternoon' THEN -1
WHEN $1 = 'afternoon' AND $2 = 'morning' THEN 1
ELSE 0
END$$;
基於此比較函數,我們定義實現比較運算符的函數:
CREATE FUNCTION tod_eq(tod, tod) RETURNS boolean IMMUTABLE LANGUAGE sql
AS 'SELECT tod_compare($1, $2) = 0';
CREATE FUNCTION tod_lt(tod, tod) RETURNS boolean IMMUTABLE LANGUAGE sql
AS 'SELECT tod_compare($1, $2) = -1';
CREATE FUNCTION tod_le(tod, tod) RETURNS boolean IMMUTABLE LANGUAGE sql
AS 'SELECT tod_compare($1, $2) <= 0';
CREATE FUNCTION tod_ge(tod, tod) RETURNS boolean IMMUTABLE LANGUAGE sql
AS 'SELECT tod_compare($1, $2) >= 0';
CREATE FUNCTION tod_gt(tod, tod) RETURNS boolean IMMUTABLE LANGUAGE sql
AS 'SELECT tod_compare($1, $2) = 1';
CREATE FUNCTION tod_ne(tod, tod) RETURNS boolean IMMUTABLE LANGUAGE sql
AS 'SELECT tod_compare($1, $2) <> 0';
現在我們可以在類型上定義運算符:
CREATE OPERATOR ~=~ (
PROCEDURE = tod_eq,
LEFTARG = tod,
RIGHTARG = tod,
COMMUTATOR = ~=~,
NEGATOR = ~<>~
);
CREATE OPERATOR ~<>~ (
PROCEDURE = tod_ne,
LEFTARG = tod,
RIGHTARG = tod,
COMMUTATOR = ~<>~,
NEGATOR = ~=~
);
CREATE OPERATOR ~<=~ (
PROCEDURE = tod_le,
LEFTARG = tod,
RIGHTARG = tod,
COMMUTATOR = ~>=~,
NEGATOR = ~>~
);
CREATE OPERATOR ~<~ (
PROCEDURE = tod_lt,
LEFTARG = tod,
RIGHTARG = tod,
COMMUTATOR = ~>~,
NEGATOR = ~>=~
);
CREATE OPERATOR ~>~ (
PROCEDURE = tod_gt,
LEFTARG = tod,
RIGHTARG = tod,
COMMUTATOR = ~<~,
NEGATOR = ~<=~
);
CREATE OPERATOR ~>=~ (
PROCEDURE = tod_ge,
LEFTARG = tod,
RIGHTARG = tod,
COMMUTATOR = ~<=~,
NEGATOR = ~<~
);
現在剩下的就是定義一個可用於定義索引的操作符類 (這需要超級用戶特權):
CREATE OPERATOR CLASS tod_ops DEFAULT FOR TYPE tod USING btree AS
OPERATOR 1 ~<~(tod,tod),
OPERATOR 2 ~<=~(tod,tod),
OPERATOR 3 ~=~(tod,tod),
OPERATOR 4 ~>=~(tod,tod),
OPERATOR 5 ~>~(tod,tod),
FUNCTION 1 tod_compare(tod,tod);
現在,我們可以定義一個使用新數據類型的表。
由於我們將tod_ops
定義為tod
類型的默認運算符類,因此我們可以創建一個簡單的唯一約束,並且基礎索引將使用我們的運算符類。
CREATE TABLE schedule (
id integer PRIMARY KEY,
day date NOT NULL,
time_of_day tod NOT NULL,
UNIQUE (day, time_of_day)
);
讓我們測試一下:
INSERT INTO schedule VALUES (1, '2018-05-01', 'morning');
INSERT INTO schedule VALUES (2, '2018-05-01', 'afternoon');
INSERT INTO schedule VALUES (3, '2018-05-02', 'anytime');
INSERT INTO schedule VALUES (4, '2018-05-02', 'morning');
ERROR: duplicate key value violates unique constraint "schedule_day_time_of_day_key"
DETAIL: Key (day, time_of_day)=(2018-05-02, morning) already exists.
PostgreSQL很酷嗎?
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