SQL JOIN 第一行与 CONCAT_WS

Question

我只想从我的用户表中引用第一个结果，但我收到以下错误

#1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '1 CONCAT_WS(' ',employeeFirstName,employeeLastName) AS courseAdded FROM vwt_user' at line 6 

我的SQL如下

CREATE OR REPLACE VIEW vwt_courses AS
SELECT tw_tblcourse.*, tw_tblprovider.providerName, CONCAT_WS(' ',vwt_userse.employeeFirstName,vwt_userse.employeeLastName) AS courseEditor, tw_tblprovider.providerAdd1, tw_tblprovider.providerAdd2, u_tbltowns.townName AS providerTown, tw_tblprovider.providerPostCode
FROM tw_tblcourse
JOIN tw_tblprovider ON tw_tblprovider.providerID = tw_tblcourse.courseProviderID
LEFT JOIN u_tbltowns ON u_tbltowns.townID = tw_tblprovider.providerTownID
JOIN vwt_users AS vwt_usersa ON vwt_usersa.userID = (SELECT TOP 1 CONCAT_WS(' ',employeeFirstName,employeeLastName) AS courseAdded FROM vwt_users WHERE userID=tw_tblcourse.courseAddedID)
LEFT JOIN vwt_users AS vwt_userse ON vwt_userse.userID = tw_tblcourse.courseEditedID

我一直在尝试关注SQL Server: How to Join to first row 的答案，但它不起作用，否则我做错了:)

我也试过了： 不适用于 MySQL，所以不起作用

CREATE OR REPLACE VIEW vwt_courses AS
SELECT tw_tblcourse.*, tw_tblprovider.providerName, CONCAT_WS(' ',vwt_usersa.employeeFirstName,vwt_usersa.employeeLastName) AS courseAdded, CONCAT_WS(' ',vwt_userse.employeeFirstName,vwt_userse.employeeLastName) AS courseEditor, tw_tblprovider.providerAdd1, tw_tblprovider.providerAdd2, u_tbltowns.townName AS providerTown, tw_tblprovider.providerPostCode
FROM tw_tblcourse
JOIN tw_tblprovider ON tw_tblprovider.providerID = tw_tblcourse.courseProviderID
LEFT JOIN u_tbltowns ON u_tbltowns.townID = tw_tblprovider.providerTownID
CROSS APPLY (SELECT  TOP 1 vwt_users.employeeFirstName, vwt_users.employeeLastName FROM vwt_users WHERE vwt_users.userID = tw_tblcourse.courseAddedID) vwt_usersa
LEFT JOIN vwt_users AS vwt_userse ON vwt_userse.userID = tw_tblcourse.courseEditedID

尝试 3： 此尝试已被接受，但由于某种原因没有限制

CREATE OR REPLACE VIEW vwt_courses AS
SELECT tw_tblcourse.*, tw_tblprovider.providerName, CONCAT_WS(' ',vwt_usersa.employeeFirstName,vwt_usersa.employeeLastName) AS courseAdded, CONCAT_WS(' ',vwt_userse.employeeFirstName,vwt_userse.employeeLastName) AS courseEditor, tw_tblprovider.providerAdd1, tw_tblprovider.providerAdd2, u_tbltowns.townName AS providerTown, tw_tblprovider.providerPostCode
FROM tw_tblcourse
JOIN tw_tblprovider ON tw_tblprovider.providerID = tw_tblcourse.courseProviderID
LEFT JOIN u_tbltowns ON u_tbltowns.townID = tw_tblprovider.providerTownID
JOIN vwt_users AS vwt_usersa ON vwt_usersa.userID = (SELECT userID FROM vwt_users WHERE userID = tw_tblcourse.courseAddedID LIMIT 1)
LEFT JOIN vwt_users AS vwt_userse ON vwt_userse.userID = (SELECT userID FROM vwt_users WHERE userID = tw_tblcourse.courseEditedID LIMIT 1)

vwt_users 示例：

╔════╦═══════╦══════╦══════╦══════╗
║ ID ║ FIRST ║ LAST ║ COMP ║ LIVE ║
╠════╬═══════╬══════╣══════╣══════╣
║  1 ║ JOHN  ║  DOE ║  1   ║  1   ║
║  1 ║ JOHN  ║  DOE ║  2   ║  1   ║
╚════╩═══════╩══════╩══════╩══════╝ 

实际字段是用户 ID、用户名、员工 ID、名字、姓氏、电子邮件、用户级别、公司 ID、公司名称、用户活动。其中，只有员工（ID、名字和姓氏）和公司（ID 和姓名）详细信息发生变化

Answer 1

TOP在MySQL中不可用，需要使用LIMIT

 SELECT  vwt_users.employeeFirstName, vwt_users.employeeLastName FROM vwt_users WHERE vwt_users.userID = tw_tblcourse.courseAddedID
 order by
 vwt_users.employeeFirstName,     vwt_users.employeeLastName
 Limit 1

当您加入 vwt_users 时，它多次具有相同的用户 ID，结果将是相同的， 相关子查询为您提供 1 行限制，但这是针对来自 vwt_users 的每个用户 ID（此处重复）。

一种方法是使用类似于您的尝试 2 的交叉连接并使用 vwt_users 删除连接

SELECT tw_tblcourse.*,
       tw_tblprovider.providerName, 
       CONCAT_WS(' ',vwt_usersa.employeeFirstName,vwt_usersa.employeeLastName) AS courseAdded,   
       CONCAT_WS(' ',vwt_userse.employeeFirstName,vwt_userse.employeeLastName) AS courseEditor 
       tw_tblprovider.providerAdd1, tw_tblprovider.providerAdd2, u_tbltowns.townName AS providerTown,  
       tw_tblprovider.providerPostCode
FROM tw_tblcourse
JOIN tw_tblprovider 
ON tw_tblprovider.providerID = tw_tblcourse.courseProviderID
LEFT JOIN u_tbltowns 
ON u_tbltowns.townID = tw_tblprovider.providerTownID
CROSS JOIN (SELECT vwt_users.employeeFirstName, vwt_users.employeeLastName FROM vwt_users WHERE userID = tw_tblcourse.courseAddedID LIMIT 1) vwt_usersa
CROSS JOIN (SELECT vwt_users.employeeFirstName, vwt_users.employeeLastName  FROM vwt_users WHERE userID = tw_tblcourse.courseEditedID LIMIT 1) vwt_userse

Answer 2

concat_ws() 不是 SQL Server 函数。它是一个 MySQL 函数。您可能可以随心所欲：

ON vwt_usersa.userID = (SELECT TOP 1 employeeFirstName + ' ' + employeeLastName AS courseAdded
                        FROM vwt_users
                        WHERE userID = tw_tblcourse.courseAddedID)

这可能无法正确处理NULLs。如果这很重要，您需要更多逻辑：

ON vwt_usersa.userID = (SELECT TOP 1 (case when employeeFirstName is NULl then employeeLastName
                                           when employeeLastName is NULL then emplyeeFirstName
                                           else employeeFirstName + ' ' + employeeLastName
                                      end) AS courseAdded
                        FROM vwt_users
                        WHERE userID = tw_tblcourse.courseAddedID)