MySQL union query (multi-table query)

One, inner connection

select *from table 1 [ inner | cross ] join table 2 [ on filter condition ] [ where query condition ];

[ inner | cross ]:

join inner join keyword (must have);

on can be omitted syntactically, but if it is omitted, it will query the Cartesian product of multiple tables;

1.select *from table 1 join table 2 [ on filter condition ] [ where query condition ];

2.select *from table 1, table 2 [ where query condition ];

3.select *from table 1 inner join table 2 [ on filter condition ] [ where query condition ];

4.select *from table 1 cross join table 2 [ on filter condition ] [ where query condition ];

(1.2 is used more; multi-table query is better to use 1)

Example 1: Query Zhang San's grades:

1. Perform inner join query (Cartesian product)

select s.*,st.* from student s join score_table st;

2. Remove invalid data (on filter condition)

select s.*,st.* from student s join score_table st on s.id=st.student_id;

3. Query Zhang San's results (where condition)

select s.*,st.* from student s join score_table st on s.id=st.student_id where s.username='Zhang San';

Example 2: Query each person's grades, subject names, personal information:

1. Join table query (three tables) select *from table 1 join table 2 join table 3;

2. Filter meaningless data in Cartesian product: select *from table 1 join table 2 [ on conditional filtering ] join table 3 [ on conditional filtering ];

select s.username,s.sn,s.mail,st.score,c.namefrom score_table st join course c on st.course_id=c.id join student s on s.id=st.student_id;

Second, outer join:

1. Left (outer) connection:

select * from table 1 left join table 2 on join condition [where condition query];

The query result of Table 1 is all data, and the query result of Table 2 is the data that overlaps with Table 1

2. Right (outer) connection

select * from table 1 right join table 2 on join condition [where condition query];

Left/right joins can be implemented with each other, just swap the order of the tables.

The difference between on and where in join table query:

1.on can be omitted in inner join, but not in outer join;

2.on has different effects in inner join and outer join;

left join...on query cannot filter out the data in the left table, while inner join on query can filter out global data.

3. On and where are different in outer joins

on filters the Cartesian product filter conditions, where filters specific businesses

Three, self-connection (self-query)

select *from table name as t1, table name as t2 where t1.id=t2.id [, …]

Example 1: Querying English Scores

(1) Find out the subject ID according to the subject name (there is only the subject ID in the score sheet, but no subject name)

(2) Self-inquiry

(3) Remove meaningless data in Cartesian product (meaningful data: the same primary key;)

(4) Set the where condition so that Table 1 only queries English scores, and Table 2 queries computer scores

(5) Set the where multi-condition query to let the English score

Four, sub-query (nested query)

select field name from

Example: Check Zhang San's classmates

Five, merge query (at least two tables)

union (merge the result set, and deduplicate, only keep one of the duplicate data)

union all (will not remove duplicate rows from the result set)

Example: query for courses with id less than 3 and name "English"

select * from course where id<3 union select * from course where name='English';

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