防止在SQL查询中重复？

Question

如果其中一个字段与其他字段之一重复，则插入新记录以使其失败时，是否有一种简便的方法？

我不希望该字段成为主键或类似的主键...

Answer 1

将列设置为唯一 。

更多关于这个

http://php.about.com/od/mysqlcommands/g/add_unique.htm

Answer 2

正如hgulyan所说，如果您希望table.col1是唯一的，请在该列上添加唯一约束。

如果您的意思是希望在列table1.col1 = table1.col2时插入操作失败，则可以在触发器中实现此操作。 请参见MYSQL创建触发器 。

要在触发器中引发异常，以使插入失败，请参阅导致INSERT失败的TRIGGER？ 可能？

就像是：

CREATE TRIGGER Employee_beforeinsert before insert
ON Employee FOR EACH ROW
    BEGIN
    IF new.age = new.age2 THEN
        DECLARE dummy INT;

        SELECT 'Your meaningful error message goes here' INTO dummy 
        FROM Employee
        WHERE Employee.id=new.id
    END IF;
END;

Answer 3

以下是标准SQL，而不是mySQL方言，但是mySQL与Standard的合规性很高，我相信您应该能够理解我的观点。

您没有发布架构或示例数据，因此我不得不猜测您的表可能是什么样的：

CREATE TABLE Mothers
(
 mother_ID INTEGER NOT NULL UNIQUE
);

CREATE TABLE Children
(
 child_ID INTEGER NOT NULL UNIQUE
);

CREATE TABLE MothersOfTwins
(
 mother_ID INTEGER NOT NULL 
    UNIQUE REFERENCES Mothers (mother_ID), 
 twin_1_child_ID INTEGER 
    REFERENCES Children (child_ID), 
 twin_2_child_ID INTEGER 
    REFERENCES Children (child_ID), 
 CHECK (twin_1_child_ID <> twin_2_child_ID)
);

INSERT INTO Mothers (mother_ID) VALUES (101), (102), (103);

INSERT INTO Children (child_ID) VALUES (551), (552), (553), (554);

INSERT INTO MothersOfTwins (mother_ID, twin_1_child_ID, twin_2_child_ID) 
VALUES 
(101, 551, 552), 
(102, 552, 551);  -- duplicate

最后一个INSERT成功，即使它失败了，即在行之间转换child_ID值也将使您想放在列上的所有UNIQUE约束都蒙混了。 我想这与您面临的问题类似。

解决此问题的一种方法是创建一个基表，该基表需要使用多行来对同级进行建模，使用带有约束条件的“出现”列来确保不超过两个同级（即双胞胎）：

DROP TABLE MothersOfTwins;

CREATE TABLE MothersOfTwinsBase
(
 mother_ID INTEGER NOT NULL
    REFERENCES Mothers (mother_ID), 
 twin_occurrence INTEGER NOT NULL
    CHECK (twin_occurrence BETWEEN 1 AND 2), 
 twin_child_ID INTEGER NOT NULL UNIQUE 
    REFERENCES Children (child_ID), 
 UNIQUE (mother_ID, twin_occurrence)
);

INSERT INTO MothersOfTwinsBase (mother_ID, twin_occurrence, twin_child_ID) 
VALUES 
(101, 1, 551), 
(101, 2, 552),
(102, 1, 553),
(103, 2, 554);

然后，您可以使用VIEW重新创建以前的基表的数据结构

CREATE VIEW MothersOfTwins
(
 mother_ID, 
 twin_1_child_ID, twin_2_child_ID
)
AS
SELECT M1.mother_ID, 
       M1.twin_child_ID AS twin_1_child_ID, 
       M2.twin_child_ID AS twin_2_child_ID
  FROM MothersOfTwinsBase AS M1 
       INNER JOIN MothersOfTwinsBase AS M2
          ON M1.mother_ID = M2.mother_ID
             AND M1.twin_occurrence = 1
             AND M2.twin_occurrence = 2
UNION ALL
SELECT M1.mother_ID, 
       M1.twin_child_ID AS twin_1_child_ID, 
       NULL AS twin_2_child_ID
  FROM MothersOfTwinsBase AS M1 
 WHERE NOT EXISTS (
                   SELECT *
                     FROM MothersOfTwinsBase AS M2
                          WHERE M1.mother_ID = M2.mother_ID
                                AND M2.twin_occurrence = 2
                  )
UNION ALL
SELECT M2.mother_ID, 
       NULL AS twin_1_child_ID, 
       M2.twin_child_ID AS twin_2_child_ID
  FROM MothersOfTwinsBase AS M2
 WHERE NOT EXISTS (
                   SELECT *
                     FROM MothersOfTwinsBase AS M1
                          WHERE M1.mother_ID = M2.mother_ID
                                AND M1.twin_occurrence = 1
                  );