Sql-->join

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.

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

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

ID    name         ID    name--    ----         --  ----1    Pirate        1    Rutabaga2    Monkey        2     Pirate3    Ninja         3    Darth Vader4    spaghetti     4    Ninja         

Let's look at the results of different joins.

SELECT * from TableA innerjoin tablebon tablea.name = tableb.name ID name ID name-- ---- -- ----1 Pirate 2 Pirate3 Ninja 4 Ninja The result set produced by the Inner join is the intersection of a and B.
SELECT * from TableA fullouter JOIN tablebon tablea.name = tableb.name ID name ID name-- ---- -- ----1 Pirate 2 Pirate2 Monkey Null null3 Ninja 4 Ninja4 spaghetti null nullnull null 1 rutabaganull null 3 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.
SELECT * from TableA leftouter JOIN TableB on tablea.name = Tableb.name ID name ID name-- ---- -- ----1 Pirate 2 Pirate2 Monkey null null3 Ninja 4 Ninja4 spaghetti null null 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.
SELECT * from TableA leftouter JOIN TableB on tablea.name = Tableb.namewhere tableb.id is null ID name ID name-- ---- -- ----2 Monkey null null4 spaghetti null NULL Produces a collection that is available in table A and not in the B table.
Select*from TableA fullouter JOIN TableB on tablea.name = Tableb.namewhere tablea.id ISnull ortableb.id ISnull ID name ID name-- ---- -- ----2 Monkey null null4 spaghetti null nullnull null 1 rutabaganull null 3 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:

SELECT * from TableA cross JOIN TableB

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.

Sql-->join