SQL是Structured Query Language(结构化查询语言)的缩写。SQL是专为数据库而建立的操作命令集，是一种功能齐全的数据库语言。在使用它时，只需要发出“做什么”的命令，“怎么做”是不用使用者考虑的。SQL功能强大、简单易学、使用方便，已经成为了数据库操作的基础，并且现在几乎所有的数据库均支持SQL。 本篇文章给大家带来的内容是关于mysql中关联变量条件修改、查询以及数据显示成一行的介绍（附代码），有一定的参考价值，有需要的朋友可以参考一下，希望对你有所帮助。一对多数据显示成一行 GROUP_CONCAT(expr) 1、涉及的表关系:teacher表、teacher_subject_rel表(教师所能教的学科表)、subject表
2、业务场景: 需要拉取所有教师的编号(teacher_no)、学科名(subject_name)。   教师表(teacher)和学科(teacher_subject_rel)是一对多关系, 往往查询出现的是同一教师多条 数据。我们希望得到每个教师一条数据 学科拼接成一条 1、基本语法 group_concat( [DISTINCT] 要连接的字段 [Order BY 排序字段 ASC/DESC] [Separator '分隔符'] )
2、例子 SELECT
t.teacher_id as '教师id',
t.teacher_no '教师编号',
(
SELECT
GROUP_CONCAT(s.subject_name)
FROM
teacher_subject_rel tsr
LEFT JOIN `subject` s ON tsr.subject_id = s.subject_id
WHERE
t.teacher_id = tsr.teacher_id
) AS '学科'
FROM
teacher t 
子查询、查询临时表、EXISTS 例子 SELECT
*
FROM
(
SELECT
o.id,
o.student_intention_id,
s. NAME,
s.area_id,
a.area_name,
s.exam_year,
o. STATUS,
CASE o. STATUS
WHEN '1' THEN
'待提交'
WHEN '2' THEN
'待指派'
WHEN '3' THEN
'已完成'
WHEN '4' THEN
'处理中'
END statusName,
CASE o.emergency_degree
WHEN '1' THEN
'正常'
WHEN '2' THEN
'紧急'
WHEN '3' THEN
'非常紧急'
END emergencyDegreeName,
o.emergency_degree,
o.update_time,
(
SELECT
first_lesson_time
FROM
jx_strategy
WHERE
jx_lesson_plan_order_id = o.id
AND STATUS IN (2, 7)
AND first_lesson_time > now()
ORDER BY
first_lesson_time ASC
LIMIT 1
) AS first_time,
(
SELECT
deal_user_id
FROM
jx_strategy
WHERE
jx_lesson_plan_order_id = o.id
AND STATUS <> 7
AND deal_user_id <> 0
ORDER BY
id DESC
LIMIT 1
) AS deal_user_id
FROM
jx_lesson_plan_order o
LEFT JOIN student s ON s.student_intention_id = o.student_intention_id
LEFT JOIN area a ON s.area_id = a.id
WHERE
o. STATUS <> 1
AND s.phone = '18501665888'
AND o.emergency_degree = 1
AND o. STATUS = 2
AND s.exam_year = '2015'
AND o.update_time >= '2018-08-14 20:28:55'
AND o.update_time <= '2018-08-14 20:28:55'
) AS a
WHERE
1 = 1
AND a.deal_user_id = 145316
AND a.first_time >= '2018-08-17 00:00:00'
AND a.first_time <= '2018-08-30 00:00:00'
AND EXISTS (
SELECT
*
FROM
jx_strategy js
WHERE
js.jx_lesson_plan_order_id = a.id
AND js. STATUS IN (2, 7)
AND js.subject_id IN (2, 3)
)
ORDER BY
a.update_time DESC
LIMIT 0,
10 update 关联变量条件修改 1、涉及的表关系: user_info表中的 id_number(身份证号) teacher表中的birth字段、 关联关系usrer_id = teacher_id
2、业务场景：获取用户身份证上的出生日期将出生日期更新在birth字段 UPDATE teacher t INNER JOIN (
SELECT t.teacher_id, t.birth, u.id_number, CONCAT(SUBSTRING(u.id_number, 7, 4), '-', SUBSTRING(u.id_number, 11, 2), '-', SUBSTRING(u.id_number, 13, 2)) as birth1, u.reg_date, t.exit_time from teacher t
INNER JOIN user_info u ON u.user_id = t.teacher_id
) info on info.teacher_id = t.teacher_id
SET t.birth = info.birth1
WHERE info.reg_date > '2018-08-20 00:00:00' and info.id_number is not NULL and (info.birth is NULL or t.birth = '') and t.is_train = 1 以上就是mysql中关联变量条件修改、查询以及数据显示成一行的介绍的详细内容，更多请关注php中文网其它相关文章！
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