Start Discovering Solved Questions and Your Course Assignments
TextBooks Included
Active Tutors
Asked Questions
Answered Questions
example of tables within a table - sqlexample obtaining cer from course and exammarkselect courseid cast table select distinct studentid mark from
max and min operator in sqlexampleselect max mark from exammark where studentid s1select min mark from exammark where studentid s1example needs no
count operator in sqlexample counting the students who have scored more than 50 in some examselect count fromselect distinct studentidfrom
aggregate operators sqlsupports all of the aggregate operators mentioned in the theory book and many more besides the syntax however involves an
semijoin and composition - sqlfor semijoin the dyadic relational operator matching defined thusr1 matching r2 where r1 and r2 are relations such that
effects of null for union - sqlthe treatment of null in invocations of except is as for union this is different from its treatment in those of not in
semidifference via not in and a subqueryselect studentidfrom iscalledwhere name devinderand studentid not in select studentidfrom isenrolledonwhere
semidifference via except and join - sqlselect from select studentidfrom iscalledwhere name devinderexcept distinct correspondingselect
semidifference and not - sqlin this section first describe the relational difference operator named minus example here shows sqls closest counterpart
union all - sqlfurther varieties of union arise when we replace the key word distinct by all in any of the foregoing examples as in example all
union without corresponding - sqlthe use of union without corresponding example is merely by omitting corresponding but only because the operands
closest approximation to relational union - sqlactually just as sql has several varieties of join it also has several varieties of union none of
union and or - sqlsql supports union explicitly but differently from the way it supports join explicitly as we have seen join is used exclusively
extension and and in sqlthe theory book gives the following simple example of relational extension in tutorial dextend iscalled add firstletter
restriction in sqlsyntaxselect distinct studentidfrom iscalledwhere name boristhe where clause operates on the result of the from clause in
restriction and and - sqlrestriction is available via the where operator and so it is in sql however by example showing how a certain simple
special cases of projectionthis section describes the identity projection r all but and the projection on no attributes r which yields tabledum
enrolment was split - sqlexample shows how relvars iscalled and isenrolledon can be derived from the original enrolment relvar using projection in
projection in sql - correct versionstudent studentid is enrolled on some courseselect distinct studentid from isenrolledonin more complicated
projection and existential quantification - sqlintuitively it might seem that projection in sql is simply a matter of specifying the required columns
using rename in combination with join - sqlexample gives pairs of ids of students having the same name by joining two renamings of iscalled example
sql outer joinselect from iscalled natural left join isenrolledonnote that adding left to an invocation of cross join has no effect unless the
explicitly specifying the join condition - sqlselect from iscalled join isenrolledonon iscalledstudentid isenrolledonstudentid now the key word
obtaining a natural join by specifying the common columnssynataxselect from iscalled join isenrolledon using studentid however a named columns join
definition of from - sqlrecall that the operand of from is denoted by a commalist each element of that commalist being a table expression optionally