Self join and joint query of MySQL

Self join

Self connect query

Self connect query , seeing the name of a thing one thinks of its function , Is to connect yourself , That is to connect and query a table many times . For self join queries , It can be internal connection query , It can also be an external connection query .

Note that the self connection must be aliased

SELECT  Field list  FROM  surface A  Alias A JOIN  surface A  Alias B ON  Conditions  ... ;

Query the names of employees and their leaders

select a.name , b.name from emp a , emp b where a.managerid = b.id;

The joint query

about union Inquire about , Is to combine the results of multiple queries , Form a new query result set .

It can be used to query multiple unrelated table data at one time

The number of columns in multiple tables of joint query must be consistent , Field types also need to be consistent .

union all All the data will be merged directly ,union The merged data will be de duplicated

SELECT  Field list  FROM  surface A ... 
UNION [ ALL ] 
SELECT  Field list  FROM  surface B ....;

Lower salary than 5000 The employees' , and Older than 50 All the employees aged 18 are found out

select * from emp where salary < 5000 
union all 
select * from emp where age > 50;

union all The result of the search , Simply merge , No weight loss .

select * from emp where salary < 5000 
union 
select * from emp where age > 50;

union The joint query , The query results will be de reprocessed .