Question

以下是我的数据库结构：

assignments(assignment_id, year, semester, title, number, weighting, handout, handin, feedback, wordcount, briefd, profile_image, sub_details, add_note, author, user_id, module_id)

requests(request_id, assignment_id, allow, requestee)

我认为这将是一项简单的任务，我希望得到来自assignments.user_id的人员，他们在请求表中有请求。我尝试了以下但是我遇到了SQL错误：

SELECT COUNT(allow) FROM requests WHERE requests.assignment_id = assignments.user_id = 31

错误：

#1054 - Unknown column 'assignments.user_id' in 'where clause'

也许我需要加入？任何帮助表示赞赏。

Answer 1

SELECT COUNT(allow) 
FROM requests WHERE requests.assignment_id = assignments.user_id AND
assignments.user_id=31

但最好使用显式JOIN

SELECT count(assignments.user_id) FROM assignments 
JOIN requests ON requests.assignment_id = assignments.assignment_id
WHERE assignments.user_id=31

Answer 2

SELECT count(assignments.user_id) 
FROM assignments 
JOIN requests ON requests.assignment_id = assignments.assignment_id 
WHERE assignments.user_id = 31

Answer 3

您应该使用on来加入2个表而不是

SELECT COUNT(allow) FROM requests ON requests.assignment_id = assignments.user_id 
         WHERE assignments.user_id = 31

看看这个

http://www.w3schools.com/sql/sql_join.asp

SQL连接无法识别连接

3 个答案: