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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
interesting properties of cross join - sqlcompare these with the interesting properties of join cross join is associative but not commutative unlike
definition of cross join - sqllet s t1 cross join t2 where t1 and t2 are table expressions optionally accompanied by range variables thennote here t
join and and in sqlin this section is all about one operator join sqls closest counterpart natural join has already been covered here we look at
relational operators and logical operatorsit prepares the ground for subsequent sections in which each specific relational operator is paired with
relational algebra - sqlit describes some operators that together constitute an algebra that is not only relationally complete but also irreducibly
existential quantification - sqlexistential quantification-stating that something is true of at least one object under consideration-can be expressed
quantification in sqlto quantify something as the theory book has it is to state its quantity to say how many of it there are for example in tutorial
logical connectives - sqlsqls extended truth tables in which the symbol for unknown appears along with the usual t and f negation not not conjunction
deriving predicates from predicates in sqlthe corresponding section in the theory book describes how predicates can be derived from predicates using
table represents an extension - sqlit describes how each tuple in a relation represents a true instantiation of some predicate and each true
substitution and instantiation - sqlit shows how null might appear in substitution for a parameter of a predicate and how it might thus participate