[英]sql unique constraint on a 2 columns combination
How can you create a unique constraint on a combination of two values in two columns.如何对两列中的两个值的组合创建唯一约束。
meaning意义
column1 column2
2 1
looking for the constraint to disallow寻找禁止的约束
column1 column2
1 2
If your database allows expressions in an index you can do something like this (ANSI SQL):如果您的数据库允许索引中的表达式,您可以执行以下操作(ANSI SQL):
CREATE UNIQUE INDEX on your_table (least(column1, column2)
, greatest(column1, column2));
Note this is a unique index not a unique constraint.请注意,这是唯一索引而不是唯一约束。 The only difference for most DBMS is that you can't have a unique index as the target of a foreign key, but otherwise they serve the same purpose.大多数 DBMS 的唯一区别是您不能将唯一索引作为外键的目标,否则它们的用途相同。
If your DBMS does not have least()
or greatest()
you can replace that using a CASE expression:如果您的 DBMS 没有least()
或greatest()
您可以使用 CASE 表达式替换它:
create unique index on your_table
(case column1 < column2 then column1 else column2 end,
case column2 > column1 then column2 else column1 end));
Looking at the documentation, found this for the ORACLE SGBD :查看文档,为 ORACLE SGBD 找到了这个:
CREATE TABLE b(
b1 INT,
b2 INT,
CONSTRAINT bu1 UNIQUE (b1, b2)
USING INDEX (create unique index bi on b(b1, b2)),
CONSTRAINT bu2 UNIQUE (b2, b1) USING INDEX bi);
Chapter "Specifying the Index Associated with a Constraint" on the page ORACLE documentation . ORACLE 文档页面上的“指定与约束关联的索引”一章。
Hop this help.跳这个帮助。
A unique constraint on 2 columns only prevents those exact 2 values being inserted (switching them is allowed):对 2 列的唯一约束仅防止插入那些确切的 2 个值(允许切换它们):
So you need A TRIGGER like this (ORACLE):所以你需要一个像这样的触发器(ORACLE):
CREATE TRIGGER trig1
BEFORE INSERT ON TAB
REFERENCING NEW AS NEW
FOR EACH ROW
DECLARE
FOUND NUMBER;
BEGIN
SELECT COUNT(1) into FOUND FROM TAB WHERE
(COLUMN1=:NEW.column2 AND COLUMN2=:NEW.column1)
OR (COLUMN1=:NEW.column1 AND COLUMN2=:NEW.column2);
IF FOUND>0 THEN
raise_application_error (-20001,'INSERT not allowed');
END IF;
END trig1;
Warning: syntax not checked.警告:未检查语法。
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