SQL Server锁定 - 避免插入重复的条目

CREATE TABLE Users (
    UserID INT PRIMARY KEY,
    UserName VARCHAR(50) NOT NULL UNIQUE
);

ALTER TABLE Users
ADD CONSTRAINT AK_Users_UserName UNIQUE (UserName);

CREATE UNIQUE INDEX idx_Users_UserName ON Users (UserName);

MERGE Users AS target
USING (SELECT @UserID, @UserName) AS source (UserID, UserName)
ON (target.UserID = source.UserID)
WHEN MATCHED THEN
    UPDATE SET UserName = source.UserName
WHEN NOT MATCHED THEN
    INSERT (UserID, UserName)
    VALUES (source.UserID, source.UserName);