Copyright 2003 Curt Hill Queries in SQL Syntax and semantics

Copyright 2003 Curt Hill Select is the basic query command The most common form is: SELECT field1, field2…fieldn FROM table1,table2…tablem WHERE condition Select f.name, c.dept, c.number, c.crhr From faculty as f, course as c, faculty_teach as ft Where f.naid=ft.naid AND ft.dept = c.dept AND ft.number = c.number

Copyright 2003 Curt Hill Syntax The statement is free form –May be on one line or many –Words, constants and strings must be complete on one line Punctuation is minimal –Commas in lists My prefererance –Select, From, Where on separate lines –One condition per line

Copyright 2003 Curt Hill Three pieces Select –Describes the resulting table From –Lists the tables that will be used to generate the final table Where –Gives the conditions or comparisons Join fields Selection comparisons –Where is optional

Copyright 2003 Curt Hill Select List fields desired This may include constants and calculations Field names may be qualified by table names if needed Corresponds to the project of algebra Use * to get all fields Many clauses to be discussed later

Copyright 2003 Curt Hill From Specify the tables Allows renaming the table for convenience in the rest of the query Form is: FROM Tab1, Tab2 AS t, Tab3 u –The AS reserved word renames for the rest of the statement –May be left out Only tables mentioned here may be accessed

Copyright 2003 Curt Hill Example Without Where Select name, degree From Faculty Select * From Faculty, Faculty_Teach –Cartesian Product

Copyright 2003 Curt Hill Where Supply conditions Compare the field name with a value –Does the selection part Compare two fields from different tables –Does the join Is actually optional but leaving it out gives the whole table or a cartesian product –Not a join

Copyright 2003 Curt Hill Where Comparisons Comparison operators –= –> –< –>= –<= –<>

Copyright 2003 Curt Hill Where boolean operators And Or Not ()

Copyright 2003 Curt Hill Student Score Example Select name, dept, course, score From Students, Grades Where Students.naid=Grades.naid Naid need qualification, since it exists in both, rest are unique From allows rename or synonyms Select name, dept, course, score From Students s, Grades as g Where S.naid=G.naid

Copyright 2003 Curt Hill Course a faculty member teaches Select name, ft.dept, course from faculty, faculty_teach ft where faculty.naid = ft.naid Put the credit hours in: Select name, ft.dept, course, crhr from faculty, faculty_teach ft, course c where faculty.naid = ft.naid AND ft.dept=c.dept AND ft.number = c.number

Copyright 2003 Curt Hill All students who got a B or better in any CS class Select name From grades, students Where score>=80 and students.naid = grades.naid And dept = ‘CS’

Copyright 2003 Curt Hill Syntax Again Reserved words and names are not sensitive to case Blanks, tabs, line feeds and other white space are ignored Layout only affects readability The order of things following Select determines the table order The order of tables following From does not matter The order of conditions does not matter

Copyright 2003 Curt Hill Designing Queries Start with the From clause –What tables are needed –Will duplicates of these be needed Make the Where next –Consider joins first –Consider selection next Finally design the Select –This is the final projection that shows the fields that you will see

Copyright 2003 Curt Hill Considering Joins How are tables connected? Two tables can be joined on any sets of fields provided: –The number and type of each set matches Names do not need to match The types are defined in the create table statement Especially look at foreign keys –Items in one that are primary in the other –Most joins are equijoins on a foreign key and primary key

Copyright 2003 Curt Hill Find all students that each faculty member teaches Select f.name, s.name from faculty as f, students s, faculty_teach ft, grades g where s.naid = g.naid AND f.naid = ft.naid And ft.dept = g.dept AND ft.number = g.number

Copyright 2003 Curt Hill Faculty members and their department and divisional chair Multiple copies are needed for multiple names Select f.name, dp.name, dv.name from faculty as f, faculty dp, faculty dv, departments dept, division div where f.dept = dept.dept AND dept.chair = dp.naid And dept.division = div.division AND div.chair = dv.naid

Copyright 2003 Curt Hill More options A select usually joins several tables creating large unique tuples The select only shows some of these fields –A projection on the larger tuple –This projection does not eliminate duplicates We now get a choice to retain or eliminate duplicates –Reserved words ALL and DISTINCT

Copyright 2003 Curt Hill All students who got a B or better in any CS class Select distinct name From grades, students Where score>=80 and students.naid = grades.naid