Rewriting SQL Queries

• Complicated by interaction of:

  • NULLs, duplicates, aggregation, subqueries.

• Guideline: Use only one “query block”, if possible.

SELECT DISTINCT *

FROM Sailors S

WHERE S.sname IN

(SELECT Y.sname

FROM YoungSailors Y)

SELECT DISTINCT S.*

FROM Sailors S,

YoungSailors Y

WHERE S.sname = Y.sname

SELECT *

FROM Sailors S

WHERE S.sname IN

(SELECT DISTINCT Y.sname

FROM YoungSailors Y)

SELECT S.*

FROM Sailors S,

YoungSailors Y

WHERE S.sname = Y.sname

• Not always possible ...

=

=

Previous slide

Next slide

Back to first slide

View graphic version