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Like SQL, QBE supports the aggregate operations AVG, COUNT, MAX, MIN, and SUM By default, these aggregate operators do not eliminate duplicates, with the exception
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of COUNT, which does eliminate duplicates To eliminate duplicate values, the variants AVGUNQ and SUMUNQ must be used (Of course, this is irrelevant for MIN and MAX) Curiously, there is no variant of COUNT that does not eliminate duplicates Consider the instance of Sailors shown in Figure 61 On this instance the following sid 22 58 44 sname dustin rusty horatio rating 7 10 7 age 450 350 350
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An Instance of Sailors
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query prints the value 383: Sailors sid sname rating age A
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Thus, the value 350 is counted twice in computing the average To count each age only once, we could specify PAVGUNQ instead, and we would get 400 QBE supports grouping, as in SQL, through the use of the G command To print average ages by rating, we could use: Sailors sid sname rating GP age A
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To print the answers in sorted order by rating, we could use GPAO or GPDO instead When an aggregate operation is used in conjunction with P, or there is a use of the G operator, every column to be printed must specify either an aggregate operation or the G operator (Note that SQL has a similar restriction) If G appears in more than one column, the result is similar to placing each of these column names in the GROUP BY clause of an SQL query If we place G in the sname and rating columns, all tuples in each group have the same sname value and also the same rating value We consider some more examples using aggregate operations after introducing the conditions box feature
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Query-by-Example (QBE) 66 THE CONDITIONS BOX
Simple conditions can be expressed directly in columns of the example tables For more complex conditions QBE provides a feature called a conditions box Conditions boxes are used to do the following: Express a condition involving two or more columns, such as R/ A > 02 Express a condition involving an aggregate operation on a group, for example, AVG A > 30 Notice that this use of a conditions box is similar to the HAVING clause in SQL The following query prints those ratings for which the average age is more than 30: Sailors sid sname rating GP age A Conditions AVG A > 30
As another example, the following query prints the sids of sailors who have reserved all boats for which there is some reservation: Sailors sid PG Id bid B1 B2 day sname rating age
Reserves
sid Id
Conditions COUNT B1 = COUNT B2
For each Id value (notice the G operator), we count all B1 values to get the number of (distinct) bid values reserved by sailor Id We compare this count against the count of all B2 values, which is simply the total number of (distinct) bid values in the Reserves relation (ie, the number of boats with reservations) If these counts are equal, the sailor has reserved all boats for which there is some reservation Incidentally, the following query, intended to print the names of such sailors, is incorrect: Sailors sid PG Id bid B1 B2 day sname P rating age
Reserves
sid Id
Conditions COUNT B1 = COUNT B2
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The problem is that in conjunction with G, only columns with either G or an aggregate operation can be printed This limitation is a direct consequence of the SQL de nition of GROUPBY, which we discussed in Section 551; QBE is typically implemented by translating queries into SQL If PG replaces P in the sname column, the query is legal, and we then group by both sid and sname, which results in the same groups as before because sid is a key for Sailors Express conditions involving the AND and OR operators We can print the names of sailors who are younger than 20 or older than 30 as follows: Sailors sid sname P rating age A Conditions A < 20 OR 30 < A
We can print the names of sailors who are both younger than 20 and older than 30 by simply replacing the condition with A < 20 AND 30 < A; of course, the set of such sailors is always empty! We can print the names of sailors who are either older than 20 or have a rating equal to 8 by using the condition 20 < A OR R = 8, and placing the variable R in the rating column of the example table
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