satyajit satyajit - 1 year ago 78
SQL Question

EXISTS vs JOIN and use of EXISTS clause

Below is the code sample:

title_id varchar(20),
title varchar(80) NOT NULL,
type char(12) NOT NULL,
pub_id char(4) NULL,
price money NULL,
advance money NULL,
royalty int NULL,
ytd_sales int NULL,
notes varchar(200) NULL,
pubdate datetime NOT NULL

insert #titles values ('1', 'Secrets', 'popular_comp', '1389', $20.00, $8000.00, 10, 4095,'Note 1','06/12/94')
insert #titles values ('2', 'The', 'business', '1389', $19.99, $5000.00, 10, 4095,'Note 2','06/12/91')
insert #titles values ('3', 'Emotional', 'psychology', '0736', $7.99, $4000.00, 10, 3336,'Note 3','06/12/91')
insert #titles values ('4', 'Prolonged', 'psychology', '0736', $19.99, $2000.00, 10, 4072,'Note 4','06/12/91')
insert #titles values ('5', 'With', 'business', '1389', $11.95, $5000.00, 10, 3876,'Note 5','06/09/91')
insert #titles values ('6', 'Valley', 'mod_cook', '0877', $19.99, $0.00, 12, 2032,'Note 6','06/09/91')
insert #titles values ('7', 'Any?', 'trad_cook', '0877', $14.99, $8000.00, 10, 4095,'Note 7','06/12/91')
insert #titles values ('8', 'Fifty', 'trad_cook', '0877', $11.95, $4000.00, 14, 1509,'Note 8','06/12/91')

stor_id char(4) NOT NULL,
ord_num varchar(20) NOT NULL,
ord_date datetime NOT NULL,
qty smallint NOT NULL,
payterms varchar(12) NOT NULL,
title_id varchar(80)
insert #sales values('1', 'QA7442.3', '09/13/94', 75, 'ON Billing','1')
insert #sales values('2', 'D4482', '09/14/94', 10, 'Net 60', '1')
insert #sales values('3', 'N914008', '09/14/94', 20, 'Net 30', '2')
insert #sales values('4', 'N914014', '09/14/94', 25, 'Net 30', '3')
insert #sales values('5', '423LL922', '09/14/94', 15, 'ON Billing','3')
insert #sales values('6', '423LL930', '09/14/94', 10, 'ON Billing','2')

SELECT title, price
FROM #titles
FROM #sales
WHERE #sales.title_id = #titles.title_id
AND qty >30)

SELECT t.title, t.price
FROM #titles t
inner join #sales s on t.title_id = s.title_id
where s.qty >30

I want to know what is the difference between the above 2 queries which gives the same result.Also want to know the purpose of EXISTS keyword and where exactly to use?

Answer Source

EXISTS is used to return a boolean value, JOIN returns a whole other table

EXISTS is only used to test if a subquery returns results, and short circuits as soon as it does. JOIN is used to extend a result set by combining it with additional fields from another table to which there is a relation.

In your example, the queries are symantically equivalent.

In general, use EXISTS when:

  • You don't need to return data from the related table
  • You have dupes in the related table (JOIN can cause duplicate rows if values are repeated)
  • You want to check existence (use instead of LEFT OUTER JOIN...NULL condition)

If you have proper indexes, most of the time the EXISTS will perform identically to the JOIN. The exception is on very complicated subqueries, where it is normally quicker to use EXISTS.

If your JOIN key is not indexed, it may be quicker to use EXISTS but you will need to test for your specific circumstance.

JOIN syntax is easier to read and clearer normally as well.

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