THE RELATIONAL MODEL 1: INTRODUCTION, QBE, AND RELATIONAL ALGEBRA - Concepts of Database Management - page 65

Division
The division process is best illustrated by considering the division of a table with two columns by a table with
a single column, which is the most common situation in which this operation is used. Consider the first two
tables shown in Figure 2-44. The first table contains two columns: OrderNum and PartNum. The second table
contains only a single column, PartNum.

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Result of dividing
OrderLine by Part OrderLine Part

OrderNum PartNum PartNum

DR93
DW11

OrderNum
21610 21608 AT94

21610 DR93

21610 DW11

21613 KL62

21614 KT03

21617 BV06

21617 CD52

21619 DR93

21623 KV29

FIGURE 2-44 Dividing one table by another

The quotient (the result of the division) is a new table with a single column named OrderNum (the col-
umn from table A that is not in table B). The rows in this new table contain those order numbers from the
OrderLine table that all the parts appearing in the Part table. For an order number to appear in the

quotient, a row in the OrderLine table must have that order number in the OrderNum column and DR93 in
the PartNum column. Also, the OrderLine table must have a row with this same order number in the
OrderNum column and DW11 in the PartNum column. It doesn

“ match ”

t matter if other rows in the OrderLine table
contain the same order number as long as the rows with DR93 and DW11 are present. With the sample data,
only order number 21610 qualifies. Thus, the result is the final table shown in the figure.

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