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Old March 27th, 2007, 04:00 AM
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Default Got problem when combine 2 tables using left join


I'm having problem with left join ...

Let's say i've these following tables

ID | year | level | m_l
147 | 2006 | dip | 20
148 | 2006 | dip | 12

ID | year | level | course_desc | m_l
147 | 2006 | dip | k | 7
147 | 2006 | dip | b | 11

(1) see course table, select sum(m_l) from course where ID=147 and year=2006 and level='dip' = 18
(2) after query from (1), if user select ID=147, year=2006, level=dip, then m_l receiving 2 or less only (0,1,2).

What i've done shown as follow:-

SELECT a.ID, a.Year, a.Level, a.m_l - b.m_l [value_left_in_course]
academic a
(SELECT SUM(m_l) [m_l] FROM course
where ID=147 and year=2006 and level='dip'
ON a.ID = b.ID
where a.ID=147 and a.year=2006 and a.level='dip'

the result is
ID | year | level | value_left_in_course
147 | 2006 | dip | 2

MY PROBLEM IS WHEN NO ROWS in course when ID=148, then i'm query as above the result is
ID | year | level | value_left_in_course
147 | 2006 | dip | (null)

How to writing SQL to changing (null), then the result shown as follow?
ID | year | level | value_left_in_course
147 | 2006 | dip | 12

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Old March 27th, 2007, 04:02 PM
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1st you need know on SELECT queries with WHERE clause you are filtering the row result of your query.
And for SELECT queries to filter an AGGREGATE FUNCTION you must filter it with HAVING clause.
SELECT a.ID As id, a.Year As yr, a.Level AS lev, SUM(a.m_l - b.m_l)
FROM academic a
HAVING a.ID = 147
AND a.year = 2006
AND a.level ='dip'


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