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wrapping and unwrapping in sqloperators wrap and unwrap in connection with attributes whose declared types are tuple types example shows how
unnest operator in sqlthe inverse operator of group is ungroup sql has an operator unnest that can be used for similar purposes but its method of
example of group by and collect operatorexample using group by and collect to obtain cer2select courseidcast collect row studentid markas row
example of groupby operatorexample how many students sat each examusing group by natural left join and coalesceselect courseid coalesce n 0 as nfrom
example of coalesce operatorexample give the total of marks for each exam simplified solutionselect courseidcoalesce select sum markfrom exammark as
example of cast operator so long as cast is used as shown we could obtain the total marks for each exam in similar fashion using sum mark as
example of using aggregation on nested tablesexample how many students sat each examwith cer as select courseid cast table select distinct studentid
using aggregation on nested tablesexample is the most direct translation of its counterpart in the theory book that can be obtained in sql but it is
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