FOR MYSQL FULL ASSOCIATION PROBLEMS, YOU CAN T USE FULL JOIN, BUT THERE IS A GOOD SOLUTION 10

Left Links There are links Full Links Left Join, Right Join, Join

If there are only 2 tables, you can leave one left and one right, and then unionize the result. If there are too many, it will be more troublesome.

For databases that do not support full joins, you can use select ,,from tab1 a left [outer] join tab2 b on =

union all

select ,,from tab1 a right [outer] join tab2 b on =

Left and right outside joins are supported, but full outer joins are not supported.

A full connection is to keep only the common attributes, which of course is supported.

The database of the project we have been working on these days is mysql

Thought about it because of your table structure design problem.

I think if you want to take it out, it's best to use a stored procedure to do it.

You can a left join b union b left join a

That is, select .from a left join b on

unionselect ..from b left join a on (association condition).

Note: The principle here is that because of the union operation, the duplicate ones will be merged.

The previous A left join b contains both A and B, and A and B without.

The following b left join a, contains both a and b, and b has a without.

union, the end result is containment.

A and B have both, and A has B without. With B there is a none.

However, if you have a left join b query that contains duplicate records, then this union method may not work.

The first table is the user, and the second table is the course

select * from a left join b on (union all

select null,null,null,null,b.* from b where not exists (select null from a where

Ay. You're so knowledgeable.

1. Natural connection select * a join b on Meaning: Use the ID of table A to match the ID of table B, and the records on the match are displayed, while those that do not match, whether the records of table A or B, are not displayed.

Left link: select * a left join b on Meaning: Use the ID of table A to match the ID of table B, the matching records are displayed, and those that do not match, all the unmatched records of table A are retained, but the unmatched records of table B are not displayed.

Right link: select * a right join b on Meaning: Use the ID of table A to match the ID of table B, the records on the match are displayed, and those that do not match, keep the records that do not match in table B, but the unmatched records of table A are not displayed.

3. Full Join: Select * A Full Join B on Meaning: Use the ID of table A to match the ID of table B, and the records on the match are displayed, while those that do not match will keep all the unmatched records of table A and table B.

4. Cartesian product: select * a, b Meaning: It means that every record in table A is combined with all the data in table B.

For example, if there are 10 records in Table A and 100 records in Table B, the records in Table A and the 100 records in Table B are combined to generate 10 x 100 records.

a full join b

What about your affiliation?

There is no correlation condition, it is 50,000 50,000 data!!

The association conditions must be written in SQL.