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Fundamental Opration

  • The Select Operation

    Select selects tuples that satisfy a given predicate. Select is denoted by a lowercase Greek sigma (), with the predicate appearing as a subscript. The argument relation is given in parentheses following the .

    For example, to select tuples (rows) of the borrow relation where the branch is "SFU", we would write

    Let Figure 3.3 be the borrow and branch relations in the banking example.

    The new relation created as the result of this operation consists of one tuple: .

    We allow comparisons using =, , <, , > and in the selection predicate.

    We also allow the logical connectives (or) and (and). For example:


    Figure 3.4: The client relation.

    Suppose there is one more relation, client, shown in Figure 3.4, with the scheme

    we might write

    to find clients who have the same name as their banker.

  • The Project Operation

    Project copies its argument relation for the specified attributes only. Since a relation is a set, duplicate rows are eliminated.

    Projection is denoted by the Greek capital letter pi (). The attributes to be copied appear as subscripts.

    For example, to obtain a relation showing customers and branches, but ignoring amount and loan#, we write

    We can perform these operations on the relations resulting from other operations.

    To get the names of customers having the same name as their bankers,

    Think of select as taking rows of a relation, and project as taking columns of a relation.

  • The Cartesian Product Operation

    The cartesian product of two relations is denoted by a cross (), written

    The result of is a new relation with a tuple for each possible pairing of tuples from and .

    In order to avoid ambiguity, the attribute names have attached to them the name of the relation from which they came. If no ambiguity will result, we drop the relation name.

    The result is a very large relation. If has tuples, and has tuples, then will have tuples.

    The resulting scheme is the concatenation of the schemes of and , with relation names added as mentioned.

    To find the clients of banker Johnson and the city in which they live, we need information in both client and customer relations. We can get this by writing

    However, the stomer.cname column contains customers of bankers other than Johnson. (Why?)

    We want rows whereclient.cname = customer.cname. So we can write

    to get just these tuples.

    Finally, to get just the customer's name and city, we need a projection:

  • <<Previous Next>>



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