free java barcode reader api Sailors sid Id sname rating <4 age Reserves D sid Id bid day in Software

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Sailors sid Id sname rating <4 age Reserves D sid Id bid day
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We can update the age of the sailor with sid 74 to be 42 years by using: Sailors sid 74 sname rating age U42
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The fact that sid is the key is signi cant here; we cannot update the key eld, but we can use it to identify the tuple to be modi ed (in other elds) We can also change the age of sailor 74 from 41 to 42 by incrementing the age value: Sailors sid 74 sname rating age U A+1
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681 Restrictions on Update Commands
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There are some restrictions on the use of the I, D, and U commands First, we cannot mix these operators in a single example table (or combine them with P) Second, we cannot specify I, D, or U in an example table that contains G Third, we cannot insert, update, or modify tuples based on values in elds of other tuples in the same table Thus, the following update is incorrect: Sailors sid sname john joe rating age U A+1 A
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This update seeks to change John s age based on Joe s age Since sname is not a key, the meaning of such a query is ambiguous should we update every John s age, and if so, based on which Joe s age QBE avoids such anomalies using a rather broad restriction For example, if sname were a key, this would be a reasonable request, even though it is disallowed
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DIVISION AND RELATIONAL COMPLETENESS *
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In Section 66 we saw how division can be expressed in QBE using COUNT It is instructive to consider how division can be expressed in QBE without the use of aggregate operators If we don t use aggregate operators, we cannot express division in QBE without using the update commands to create a temporary relation or view However,
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taking the update commands into account, QBE is relationally complete, even without the aggregate operators Although we will not prove these claims, the example that we discuss below should bring out the underlying intuition We use the following query in our discussion of division: Find sailors who have reserved all boats In 4 we saw that this query can be expressed in DRC as: { I, N, T, A | I, N, T, A Sailors B, BN, C Boats ( Ir, Br, D Reserves(I = Ir Br = B))} The quanti er is not available in QBE, so let us rewrite the above without : { I, N, T, A | I, N, T, A Sailors B, BN, C Boats ( Ir, Br, D Reserves(I = Ir Br = B))} This calculus query can be read as follows: Find Sailors tuples (with sid I) for which there is no Boats tuple (with bid B) such that no Reserves tuple indicates that sailor I has reserved boat B We might try to write this query in QBE as follows:
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Sailors
sid Id
sname P S
rating
Boats
bid B
bname
color
Reserves
sid Id
bid B
This query is illegal because the variable B does not appear in any positive row Going beyond this technical objection, this QBE query is ambiguous with respect to the ordering of the two uses of It could denote either the calculus query that we want to express or the following calculus query, which is not what we want: { I, N, T, A | I, N, T, A Sailors Ir, Br, D Reserves ( B, BN, C Boats(I = Ir Br = B))} There is no mechanism in QBE to control the order in which the operations in a query are applied (Incidentally, the above query nds all Sailors who have made reservations only for boats that exist in the Boats relation) One way to achieve such control is to break the query into several parts by using temporary relations or views As we saw in 4, we can accomplish division in
Query-by-Example (QBE)
two logical steps: rst, identify disquali ed candidates, and then remove this set from the set of all candidates In the query at hand, we have to rst identify the set of sids (called, say, BadSids) of sailors who have not reserved some boat (ie, for each such sailor, we can nd a boat not reserved by that sailor), and then we have to remove BadSids from the set of sids of all sailors This process will identify the set of sailors who ve reserved all boats The view BadSids can be de ned as follows: Sailors sid Id sname rating age Reserves sid Id bid B day
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