Joe Celko s SQL for Smarties - Advanced SQL Programming P36. In the SQL database community, Joe Celko is a well-known columnist and purveyor of valuable insights. In Joe Celko's SQL for Smarties: Advanced SQL Programming, he picks up where basic SQL training and experience leaves many database professionals and offers tips, techniques, and explanations that help readers extend their capabilities to top-tier SQL programming. Although Celko denies that the book is about database theory, he nevertheless alludes to theory often to buttress his practical points. This title is not for novices, as the author points out. Instead, its intended audience. | 322 CHAPTER 17 THE SELECT STATEMENT 2 Smyth F 17 8 A 3 Smoot F 16 1 A 3 Smoot F 16 2 B 3 Smoot F 16 3 C 3 Smoot F 16 4 D 3 Smoot F 16 5 A 3 Smoot F 16 6 A 3 Smoot F 16 7 A 3 Smoot F 16 8 A 4 Adams F 17 1 A 4 Adams F 17 2 B 4 Adams F 17 3 C 4 Adams F 17 4 D 4 Adams F 17 5 A 4 Adams F 17 6 A 4 Adams F 17 7 A 4 Adams F 17 8 A 5 Jones M 16 1 A 5 Jones M 16 2 B 5 Jones M 16 3 C 5 Jones M 16 4 D 5 Jones M 16 5 A 5 Jones M 16 6 A 5 Jones M 16 7 A 5 Jones M 16 8 A 6 Celko M 17 1 A 6 Celko M 17 2 B 6 Celko M 17 3 C 6 Celko M 17 4 D 6 Celko M 17 5 A 6 Celko M 17 6 A 6 Celko M 17 7 A 6 Celko M 17 8 A 7 Vennor F 16 1 A 7 Vennor F 16 2 B 7 Vennor F 16 3 C 7 Vennor F 16 4 D 7 Vennor F 16 5 A 7 Vennor F 16 6 A 7 Vennor F 16 7 A 7 Vennor F 16 8 A SELECT and JOINs 323 8 Murray M 18 1 A 8 Murray M 18 2 B 8 Murray M 18 3 C 8 Murray M 18 4 D 8 Murray M 18 5 A 8 Murray M 18 6 A 8 Murray M 18 7 A 8 Murray M 18 8 A There are two predicates in the WHERE. The first predicate grade A needs only the Students table. In fact an optimizer in a real SQL engine would have removed those rows in the Students table that failed the test before doing the cross join. The second predicate is which requires both tables and the constructed row. Predicates that use values from two tables are called join conditions for obvious reasons. Now remove the rows that do not meet the conditions. After the WHERE clause the result table looks like this Cross Join after WHERE clause Students Gradebook stud_nbr stud_name sex age stud_nbr grade 1 Smith M 16 I 1 A 5 Jones M 16 I 5 A 6 Celko M 17 I 6 A 7 Vennor F 16 I 7 A 8 Murray M 18 I 8 A We have a group by clause that will group the working table by sex thus by sex Students Gradebook stud_nbr stud_name sex age stud_nbr grade 1 Smith M 16 I 1 A sex M group 5 Jones M 16 5 A 6 Celko M 17 6 A 8 Murray M 18 8 A 7 Vennor F 16 1 7 A sex F group 324 CHAPTER 17 THE SELECT STATEMENT And the aggregate functions in the select clause are .
