Question

我想在我的数据库表中添加一个约束，以便字符串＆＃34; username＆＃34;只能输入特定列最多10次。这可能吗？

我正在使用ms sql server。

Answer 1

您可以使用AFTER触发器UPDATE/INSERT：

CREATE TABLE tab(col NVARCHAR(100));

CREATE TRIGGER trg_tab
ON tab
AFTER INSERT, UPDATE
AS
 IF (SELECT COUNT(*) FROM tab WHERE col = 'username') > 10
 ROLLBACK;

INSERT INTO tab(col)
VALUES ('username'), ('username'), ('username'), ('username'),
       ('username'), ('username'), ('username'), ('username'),
       ('username'),('username');

INSERT INTO tab(col)
VALUES ('username');  

/* The transaction ended in the trigger. The batch has been aborted. */

或使用SCALAR FUNCTION和CHECK约束：

<强> SQLFiddleDemo2

CREATE TABLE tab(col NVARCHAR(100));

CREATE FUNCTION dbo.CheckFunction()
RETURNS INT
AS
BEGIN
    RETURN (SELECT COUNT(*) FROM tab WHERE col = 'username')
END;

ALTER TABLE tab
ADD CONSTRAINT chkRowCount CHECK (dbo.CheckFunction() <= 10 );

INSERT INTO tab(col)
VALUES ('username'), ('username'), ('username'), ('username'),
       ('username'), ('username'), ('username'), ('username'),
       ('username'),('username');

INSERT INTO tab(col)
VALUES ('username');  

/* The INSERT statement conflicted with the CHECK constraint "chkRowCount".
 The conflict occurred in database "db_6_34d5e", table "dbo.tab". */

Answer 2

在表格上创建AFTER INSERT触发器。像这样的事情

create trigger LimitTable
on YourTableToLimit
after insert
as
    declare @tableCount int
    select @tableCount = Count(*)
    from YourTableToLimit

    if @tableCount > 50
    begin
        rollback
    end
go

约束只允许10个字符串实例＆＃34; username＆＃34;

2 个答案: