How can I write following query using left join here is my Sample Query
SELECT * FROM STUDENT st WHERE st.stdid not IN ( SELECT csch.stid FROM courseSchedule csch, section sctn WHERE cschno.sectno = sctn.sectno AND cschno.college= sctn.college AND sctn.college = st.college AND sctn.year = 2013 and sctn.semester = 'spring' ) AND st.college = 1 TO move the NOT IN subquery to a LEFT JOIN, do left join on the NOT IN key criteria, and instead, check for the Right table KEY as NULL. Note that you can also convert the WHERE join in the subquery to an ANSI join as well:
SELECT st.* FROM STUDENT st LEFT JOIN courseSchedule csch ON st.stdid = csch.stid INNER JOIN section sctn ON cschno.sectno = sctn.sectno AND cschno.college= sctn.college AND sctn.college = st.college AND sctn.year = 2013 AND sctn.semester = 'spring' WHERE st.college = 1 AND csch.stid IS NULL; -- "Not IN" 
LEFT JOIN criteria, and in the outer WHERE clause which you need to be wary of.RHS table columns will be NULL because of the Left join, which is detected in the WHERE clause.@Waqas, I would use EXISTS for this (actually NOT EXISTS) as a commenter suggested above:
SELECT * FROM STUDENT st WHERE st.college = 1 AND NOT EXISTS ( SELECT 1 FROM courseSchedule csch, section sctn WHERE csch.stid = st.stdid AND csch.sectno = sctn.sectno AND csch.college = sctn.college AND sctn.year = 2013 AND sctn.semester = 'spring' AND sctn.college = st.college ) I notice you also have a typo in your query (the alias for courseSchedule is csch, not cschno).
Hope this helps.
