Various join joins for SQL

Source: Internet
Author: User
Tags joins

For SQL joins, learning may be a bit confusing. We know that the join syntax for SQL has a lot of inner, outer, left, and sometimes it's not very clear what the result set looks like for a select. There is an article on Coding horror (it is not clear why Coding horror was also the wall) through the Venturi diagram Venn diagrams explained the join of SQL. I feel clear and understandable, turn around.

Suppose we have two tables, table A is the one on the left and table B is the one on the right. What is the Venetian Macao?

Each of them has four records, of which two records are the same, as follows:

1 id nameid  name
2 -- ----       --  ----
3 1  Pirate     1   Rutabaga
4 2  Monkey     2   Pirate
5 3  Ninja      3   Darth Vader
6 4  Spaghetti  4   Ninja

Let's look at the results of different joins.

3 id  nameid   name
4 --  ----       --   ----
5 1   Pirate     2    Pirate
6 3   Ninja      4    Ninja

Inner Join
The resulting set of results is the intersection of a and B.

03 id    nameid    name
04 --    ----       --    ----
05 1     Pirate     2     Pirate
06 2     Monkey     nullnull
07 3     Ninja      4     Ninja
08 4     Spaghetti  nullnull
09 nullnull1     Rutabaga
10 nullnull3     Darth Vader

The full outer join produces a and B's set. It is important to note, however, that for records that do not have a match, NULL is the value.

2 id  nameid    name
3 --  ----       --    ----
4 1   Pirate     2     Pirate
5 2   Monkey      nullnull
6 3   Ninja      4     Ninja
7 4   Spaghetti   nullnull

The left outer join produces a full set of table A, whereas a match in B table has a value, and no match is substituted with a null value.

1 select  from  tablea   left outer  join  tableb  on tablea. name  = TableB. name where  is  null
2 id  nameid     name
3 --  ----       --     ----
4 2   Monkey  nullnull
5 4   Spaghetti   nullnull

Produces a collection that is available in table A and not in the B table.

1 select  from  tablea   full outer  join  tableb  on tablea. name  = TableB. name where  is  null  or   is  null
2 id    nameid    name
3 --    ----       --    ----
4 2     Monkey    nullnull
5 4     Spaghetti nullnull
6 nullnull1     Rutabaga
7 nullnull3     Darth Vader

produce datasets that do not appear in both A and B tables.

It is also necessary to register that we also have a cross join of "cross-set", which is not represented by Wenshitu because it is a n*m combination of the data of table A and table B, that is, the Cartesian product. The expression is as follows:


This Cartesian product produces 4 x 4 = 16 records, which, in general, are seldom used in this syntax. But we have to be careful, if you do not use nested SELECT statements, the general system will produce a Cartesian product and then filter. This is very dangerous for performance, especially when the table is very large.

Various join joins for SQL

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