MYSQL uses Union to merge the data of two tables and display them

Using the UNION Operator

union : Used to connect the results of two or more SELECT statements into one result set. Multiple SELECT statements will remove duplicate data.

Using the union operator will retrieve the same data from multiple tables once. If you want to display the values ​​in Table 1 and Table 2 completely, you can use union all.

Demo

Friends, create a table by yourself.

The data in Table 1 are as follows:

The data in Table 2 are as follows:

OK, the table data has been created, with a total of five records. Next, let's take a look at the use of union and union all.

Use union to see the effect:

select t1.id id, t1.name name, t1.description description,t1.create_time time from table1 t1
UNION
select t2.id id, t2.name name, t2.description description,t2.create_date time from table2 t2 

We can see that using union will only find four pieces of data. If two of them are identical data, only one will be displayed.

Use union all to see the effect:

select t1.id id, t1.name name, t1.description description,t1.create_time time from table1 t1
UNION ALL
select t2.id id, t2.name name, t2.description description,t2.create_date time from table2 t2 

Use union all to find out 5 pieces of data. PS: The same data will also be queried.

expand:

In order to distinguish which table the data is in, we can do this

select t1.id id, t1.name name, t1.description description,t1.create_time time,'table1' type from table1 t1
UNION ALL
select t2.id id, t2.name name, t2.description description,t2.create_date time,'table2' type from table2 t2 

Sort the data in the two tables by time

select t3.* from (select t1.id id, t1.name name, t1.description description,t1.create_time time,'table1' type from table1 t1
UNION ALL
select t2.id id, t2.name name, t2.description description,t2.create_date time,'table2' type from table2 t2) t3 order by t3.time desc 

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