What is the difference between join and union.
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Answer Posted / blue shark
JOIN:
-----
A Join is used for displaying columns with the same or
different names from different tables. The output displayed
will have all the columns shown individually. i.e. The
columns will be aligned next to each other.
UNION:
------
The UNION set operator is used for combining data from two
tables which have columns with the same datatype.
When a UNION is performed the data from both tables will be
collected in a single column having the same datatype.
For eg:
See the two tables shown below:
Table t1
Articleno article price manufacturer_id
1 hammer 3 $ 1
2 screwdriver 5 $ 2
Table t2
manufacturer_id manufacturer
1 ABC Gmbh
2 DEF Co KG
Now for performing a JOIN type the query shown below
SELECT articleno, article, manufacturer
FROM t1 JOIN t2 ON (t1.manufacturer_id =
t2.manufacturer_id);
articelno article manufacturer
1 hammer ABC GmbH
2 screwdriver DEF Co KG
That is a join.
UNION means that you have to tables or resultset with the
same amount and type of columns and you add this to
tables/resultsets together. Look at this example:
Table year2006
Articleno article price manufacturer_id
1 hammer 3 $ 1
2 screwdriver 5 $ 2
Table year2007
Articleno article price manufacturer_id
1 hammer 6 $ 3
2 screwdriver 7 $ 4
SELECT articleno, article, price, manufactruer_id
FROM year2006
UNION
SELECT articleno, article, price, manufacturer_id
FROM year2007
articleno article price manufacturer_id
1 hammer 3 $ 1
2 screwdriver 5 $ 2
1 hammer 6 $ 3
2 screwdriver 7 $ 4
Hope this helps!
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