copyright 2003 curt hill queries in sql syntax and semantics

Copyright 2003 Curt Hill

Queries in SQL

Syntax and semantics


Select is the basic query command

• The most common form is:SELECT field1, field2…fieldnFROM table1,table2…tablemWHERE condition

• Select f.name, c.dept, c.number, c.crhrFrom faculty as f, course as c, faculty_teach as ftWhere f.naid=ft.naid AND ft.dept = c.dept AND ft.number = c.number


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


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


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


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


Example Without Where

• Select name, degreeFrom Faculty

• Select *From Faculty, Faculty_Teach– Cartesian Product


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


Where Comparisons

• Comparison operators– =– >– <– >=– <=– <>


Where boolean operators

• And• Or• Not• ()


Student Score Example• Select name, dept, course, score

From Students, GradesWhere 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 gWhere S.naid=G.naid


Course a faculty member teaches

• Select name, ft.dept, coursefrom faculty, faculty_teach ftwhere faculty.naid = ft.naid

• Put the credit hours in:• Select name, ft.dept, course, crhr

from faculty, faculty_teach ft, course cwhere faculty.naid = ft.naid AND ft.dept=c.dept AND ft.number = c.number


All students who got a B or better in any CS class

• Select nameFrom grades, studentsWhere score>=80 and students.naid = grades.naid And dept = ‘CS’


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


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


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


Find all students that each faculty member teaches

• Select f.name, s.namefrom faculty as f, students s, faculty_teach ft, grades gwhere s.naid = g.naid AND f.naid = ft.naid And ft.dept = g.dept AND ft.number = g.number


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 divwhere f.dept = dept.dept AND dept.chair = dp.naid And dept.division = div.division AND div.chair = dv.naid


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


All students who got a B or better in any CS class

• Select distinct nameFrom grades, studentsWhere score>=80 and students.naid = grades.naid

copyright 2003 curt hill queries in sql syntax and semantics

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