Php mysql选择多任务-连接
假设我有以下表格Php mysql选择多任务-连接,php,mysql,sql,relational-division,Php,Mysql,Sql,Relational Division,假设我有以下表格 user_table id username 1 abc 2 def 3 ghij courses_table id title 1 csc 2 math 3 syn user_courses user_id course_id 2 1 1 3 2 3 我想选择参加课程1和课程3的用户名, 至少不是1或3,我指的是1和3 我尝试了以下mysq
user_table
id username
1 abc
2 def
3 ghij
courses_table
id title
1 csc
2 math
3 syn
user_courses
user_id course_id
2 1
1 3
2 3
我想选择参加课程1和课程3的用户名,
至少不是1或3,我指的是1和3
我尝试了以下mysql查询
SELECT DISTINCT u.* FROM user_table as u LEFT JOIN user_courses as uc ON uc.user_id = u.id WHERE uc.course_id = 1 AND uc.course_id=3;
SELECT DISTINCT u.* FROM user_table as u LEFT JOIN user_courses as uc ON uc.user_id = u.id WHERE uc.course_id IN (1,3);
SELECT DISTINCT u.* FROM user_table as u LEFT JOIN user_courses as uc ON uc.user_id = u.id WHERE uc.course_id IN (1,3) AND uc.user_id = u.id ;
第一个和第三个查询执行时没有显示结果,第二个查询显示至少拥有课程id 1或3的所有用户
如果您想知道我为什么使用左连接,这是因为我需要连接表的结果,上面的代码行只是一个示例,我使用左连接从大约9个表中获取数据
需要帮忙吗?谢谢
SELECT DISTINCT u.* FROM user_table as u LEFT JOIN user_courses as uc ON uc.user_id = u.id WHERE uc.course_id IN( 1,3) AND uc.user_id = 2 ";
这显示我想要的结果,它的输出“def”,
但是我不能将用户id用作静态值(本例中的数字2)这个问题称为
关系分割
SELECT a.id, a.username
FROM user_table a
INNER JOIN user_courses b
ON a.id = b.user_ID
WHERE b.course_ID IN (1,3)
GROUP BY a.id, a.username
HAVING COUNT(*) = 2
课程\u ID
对于每个用户来说都不是唯一的,考虑到用户已经重修了课程,则需要一个DISTINCT
关键字来咨询唯一的课程
SELECT a.id, a.username
FROM user_table a
INNER JOIN user_courses b
ON a.id = b.user_ID
WHERE b.course_ID IN (1,3)
GROUP BY a.id, a.username
HAVING COUNT(DISTINCT b.course_ID) = 2
╔════╦══════════╗
║ ID ║ USERNAME ║
╠════╬══════════╣
║ 2 ║ def ║
╚════╩══════════╝
请试试这个:
SELECT
U.id,
U.username
FROM
user_courses UC
INNER JOIN user_table U
ON UC.`user_id` = U.`id`
WHERE UC.`course_id` = 1
OR UC.`course_id` = 3
GROUP BY U.`id`
HAVING COUNT(*) > 1