Database learning – select (multi table joint query) [easy to understand]

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Multi table joint query

Joint query of multiple tables can be realized through join operation , The connection operation can be realized through the selection operation after the generalized Cartesian product .

Select Multi table joint query statement

Select Name [[, Name ]…] from Table name 1, Table name 2,,… Where Query criteria ;

have access to as Is a table or field （ attribute ） Take the alias ;

Select Name [[, Name ]…] from Table name 1 [as] Alias 1, Table name 2 [as} Alias 2,,… Where Query criteria ;

Select Name 1 [as] Column alias 1 [[, Name 2 [as] Column alias 2]…] from Table name 1 [as] Table alias 1, Table name 2 [as] Table alias 2,,… Where Query criteria ;

“[]” Indicates that the contents can be omitted ;

Query conditions should include connection conditions , Equivalent connection can be realized through different connection conditions 、 Unequal value connection and other connections .

Example ：

The original table data ：

Student list student

The teacher table teacher：

The curriculum course：

Course selection table student_course：

1 Press “0001” The student ID of all students will be displayed in order from high to low 、 full name 、 achievement （ Two meter connection ）;

SELECT
	student.student_id,
	student.student_name,
	student_course.score
FROM
	student,
	student_course
WHERE
	student.student_id = student_course.student_id
AND student_course.course_id = '0001'
ORDER BY
	student_course.score DESC;

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When two （ Multiple ） There are the same fields in the table （ attribute , Name ） when , Use the table name （ Or table alias ）. The form of column name distinguishes the query content from the condition content .

Running results ：

2 Press “ Advanced mathematics ” The names of all students are displayed in order of course grades 、 Student number 、 fraction 、 Course name （ Three meter connection ）;

SELECT
	st.student_id,
	st.student_name,
	sc.score,
	co.course_name
FROM
	student AS st,
	student_course AS sc,
	course AS co
WHERE
	st.student_id = sc.student_id
AND sc.course_id = co.course_id
AND co.course_name = ' Advanced mathematics '
ORDER BY
	sc.score DESC;

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Running results ：

3 Query any two teachers with salary differences （ Single table connection query ）;

SELECT
	t1.teacher_id AS ' Faculty number 1',
	t1.teacher_name AS ' Teacher's name 1',
	t2.teacher_id AS ' Faculty number 2',
	t2.teacher_name AS ' Teacher's name 2'
FROM
	teacher t1,
	teacher t2
WHERE
	t1.teacher_salary > t2.teacher_salary;

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Running results ：

（ More data , Only a part is shown here ）

4 Query has been learned “0001” I've learned “0002” Student numbers of all students in course No ;

SELECT
	sc1.student_id ' Student number '
FROM
	student_course sc1,
	student_course sc2
WHERE
	sc1.student_id = sc2.student_id
AND sc1.course_id = '0001'
AND sc2.course_id = '0002';

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Running results ：

5 Inquire about “0001” Course No “0002” Student ID names of all students with low grades in course No ;

SELECT
	st.student_id ' Student number ',
	st.student_name ' full name '
FROM
	student_course sc1,
	student_course sc2,
	student st
WHERE
	sc1.student_id = sc2.student_id
AND sc1.student_id = st.student_id
AND sc1.course_id = '0001'
AND sc2.course_id = '0002'
AND sc1.score < sc2.score;

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Running results ：

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