primary vs candidate vs composite key unique.
one table one primary key they value not null if you make other primary key in same table this is composite key this is allow null and blank value.
primary key is candidate key if you make one table more primary key
unique is one table more then one this value is null accept one time
PK can easily be ‘auto incremented’ by the database.This doesn’t hold for a composite key.
php oops question answer
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.
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.
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
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 http://stackoverflow.com/questions/38549/difference-between-inner-and-outer-joins?rq=1
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