The exists
keyword can be used in that way, but really it's intended as a way to avoid counting:
--this statement needs to check the entire table
select count(*) from [table] where ...
--this statement is true as soon as one match is found
exists ( select * from [table] where ... )
This is most useful where you have if
conditional statements, as exists
can be a lot quicker than count
.
The in
is best used where you have a static list to pass:
select * from [table]
where [field] in (1, 2, 3)
When you have a table in an in
statement it makes more sense to use a join
, but mostly it shouldn't matter. The query optimiser should return the same plan either way. In some implementations (mostly older, such as Microsoft SQL Server 2000) in
queries will always get a nested join plan, while join
queries will use nested, merge or hash as appropriate. More modern implementations are smarter and can adjust the plan even when in
is used.
Assuming you're joining on columns with no duplicates, which is a very common case:
An inner join of A and B gives the result of A intersect B, i.e. the inner part of a Venn diagram intersection.
An outer join of A and B gives the results of A union B, i.e. the outer parts of a Venn diagram union.
Examples
Suppose you have two tables, with a single column each, and data as follows:
A B
- -
1 3
2 4
3 5
4 6
Note that (1,2) are unique to A, (3,4) are common, and (5,6) are unique to B.
Inner join
An inner join using either of the equivalent queries gives the intersection of the two tables, i.e. the two rows they have in common.
select * from a INNER JOIN b on a.a = b.b;
select a.*, b.* from a,b where a.a = b.b;
a | b
--+--
3 | 3
4 | 4
Left outer join
A left outer join will give all rows in A, plus any common rows in B.
select * from a LEFT OUTER JOIN b on a.a = b.b;
select a.*, b.* from a,b where a.a = b.b(+);
a | b
--+-----
1 | null
2 | null
3 | 3
4 | 4
Right outer join
A right outer join will give all rows in B, plus any common rows in A.
select * from a RIGHT OUTER JOIN b on a.a = b.b;
select a.*, b.* from a,b where a.a(+) = b.b;
a | b
-----+----
3 | 3
4 | 4
null | 5
null | 6
Full outer join
A full outer join will give you the union of A and B, i.e. all the rows in A and all the rows in B. If something in A doesn't have a corresponding datum in B, then the B portion is null, and vice versa.
select * from a FULL OUTER JOIN b on a.a = b.b;
a | b
-----+-----
1 | null
2 | null
3 | 3
4 | 4
null | 6
null | 5
Best Answer
SQL
is a query language to operate on sets.It is more or less standardized, and used by almost all relational database management systems: SQL Server, Oracle, MySQL, PostgreSQL, DB2, Informix, etc.
PL/SQL
is a proprietary procedural language used by OraclePL/pgSQL
is a procedural language used by PostgreSQLTSQL
is a proprietary procedural language used by Microsoft in SQL Server.Procedural languages are designed to extend SQL's abilities while being able to integrate well with SQL. Several features such as local variables and string/data processing are added. These features make the language Turing-complete.
They are also used to write stored procedures: pieces of code residing on the server to manage complex business rules that are hard or impossible to manage with pure set-based operations.