Why do we create multiple aliases to a single table?

Question

<h4>{color:#0000ff}Hi,{color}</h4>
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<h4>{color:#0000ff} Why do we create multiple aliases to a single table in the same query?{color}</h4>
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<h4>{color:#0000ff}{color:#000000}For Ex: {color}{color}</h4>
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{color:#000000}select name,address,phone, from emp e, emp e1{color}</h4>
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<h4>{color:#000000}where e.empid = e1.empid;{color}</h4>
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{color:#0000ff}ofcourse the above query's where condition is not correct, but i am giving as an example only{color}</h4>
<h4>{color:#0000ff}I have seen similar kind of queries where a single table name is aliased 10 times in the same query{color}</h4>
<h4>{color:#0000ff}can someone help me to understand the logic behind it?{color}</h4>
<h4>{color:#0000ff}Thanks in advance{color}</h4>
<h4>{color:#0000ff}greddy.{color}</h4>
Edited by: greddy on Oct 24, 2008 2:46 AM

Answer 1

Hi,
When you say
FROM    emp  e
,       emp  e1you are using two copies of the same table.
Can you imagine using two copies of the same book at the same time?
Say you see a word (like "hyrax") that you don't know. You might look up that word in the dictionary.
The English sentence "Open the dictionary to the page containing the new word." is like this SQL query:
FROM    dictionary
WHERE   :unknown_word  BETWEEN low_guide_word and high_guide_wordWhat if you saw that word while reading a book (let's call it book_a).
You might want to leave book_a open, and place the dictionary beside book_a, so you can easily compare how the word is used in book_a with the definition in the dictionary.
This corresponds to joining two tables in SQL:
FROM    book_a     
JOIN    dictionary  ON unknown_word BETWEEN low_guide_word AND high_guide_word
WHERE   book_a.page  = whatever  -- NOTE: both tables have a column called pageor you can use aliases for one or both of the tables
FROM    book_a      a
JOIN    dictionary  d  ON a.unknown_word BETWEEN d.low_guide_word AND d.high_guide_word
WHERE   a.page  = whatever  -- NOTE: both tables have a column called pageNow, imagine that book_a is itself the dictionary. Say you saw the word "hyrax" while you were looking up another word, "ibex".
You can do a side-by-side comparison, as above, if you have two copies of the dictionary. Leave one copy open to "ibex", and open the other copy to "hyrax".
This corresponds to a self-join in SQL:
FROM    dictionary  a
JOIN    dictionary  d  ON a.unknown_word BETWEEN d.low_guide_word AND d.high_guide_word
WHERE   a.page  = whatever  -- NOTE: both tables have a column called pageNotice that this last bit of code is identical to the previous one, except that book_a is replaced with dictionary.
In this case, you must you a table alias, at least for one copy. It wouldn't make any sense to say "Leave the dictionary open to "ibex" and open the dictionary to "hyrax". You have to have some unique way of referring to each copy, such as "Leave the dictionary on my left open to "ibex" and open the dictionary on my right to "hyrax"."