macro excel ean 128 Use of Workspaces in Software

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Use of Workspaces
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We use again the employee relations from Fig 7-10: Employee 2: RELATION Supervision : RELATION name : age, , experience; super, sub : years; Bosses(b name, b age) = SELECT name, age FROM Employee 2, Supervision WHERE name = super; Workers(w super, w age) = SELECT super, age FROM Employee 2, Supervision WHERE name = sub; SELECT b name FROM Bosses, Workers WHERE b name = w super;
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Example 9-8
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Database Implementation Derived Values in Workspaces
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An operation which can create a new attribute would be the calculation of the weight in stock for the various parts, given the Supply and Parts relations of Fig 7-16 We have to use tuple variables to distinguish the attributes Parts: RELATION Supply : RELATION s id, p id : quantity ; p id : name,size,weight,; Stockweight(p id, pounds) = SELECT partp id, supquantity * partweight FROM Parts part, Supply sup WHERE partp id = supp id; The relation Stockweight contains as many tuples as the Supply relation (7) since the join implied here of Parts and Supply uses an ownership connection from Parts to Supply in the model Some part numbers are repeated since they came from di erent Suppliers
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Workspaces are useful when the result relations are further processed For instance, to obtain a commercial-looking output a Total tuple should be appended to the list created above Example 9-9 uses a SEQUEL statement, since SQL does not permit the same table to be used as the destination and source of an INSERT
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Example 9-9
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Inserting a SUM into the Workspace
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We will augment this workspace with a single tuple for the total weight Using the function SUM and the INSERT INTO operation from SEQUEL:
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INSERT INTO Stockweight(p id, pounds) : ("Total", SUM( SELECT pounds FROM Stockweight) ); Supplementary capabilities have been proposed for the relational sublanguage SQUARE SQUARE as de ned uses a two-dimensional notation to avoid workspaces and tuple variables To demonstrate the features of SQUARE in the notation used here, a relation is created which contains the total weight of each part type
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Example 9-10
Inserting SUMs of Groups into the Workspace
Stockweight(p id, pounds) <SELECT DISTINCT partp id, SUM ( SELECT supquantity * part grpweight ) WHERE partp id = supp id FROM Parts as part, Supply as sup ; Now only one tuple per p id is left We can still insert a total tuple; the result relation is shown in Fig 9-3
Sec 9-2
Integrity Constraints
Relational Calculus Implementation
In order to maintain a database correctly, the constraints implied by the connections de ned in the database model still have to be imposed
For instance, without the maintenance of the ownership connection among the relations we might not be sure that all p ids in Supply were listed in Parts If any p ids are missing from Parts, the join caused by the WHERE clause of Example 9-8 will fail to include those parts in the result and in the nal pounds calculations
In most relational systems such constraints are the responsibility of the user; a basic relational schema does not provide for speci cation of update constraints Two alternatives have to be considered, maintenance of integrity during update or consideration of lack of integrity during retrieval Even if most update commands are executed with care, and include WHERE clauses to assure that constraints are obeyed, no guarantee can be given by the system that there are no inconsistencies This means that queries have to formulated with great care There is, for instance, no guarantee that attributes along a connection will match, and hence a chance that a join will fail to retrieve expected data
In Example 9-3 we wrote the WHERE clause permitting arbitrary substrings: job LIKE "%Welder%" to ensure that the query will also retrieve Hare, the Asst Welder We found that the prior query in Example 9-2, using LIKE "Welder" for an exact match along the presumed reference connection, did not retrieve that record In this case the reference connection to the table de ning the domain of jobs was not maintained for this attribute
Such maintenance constraints have to be added explicitly to update commands or queries have to be written to take care of approximate matches to reduce the chance of missing records that should be connected
Integrity Constraint Assertions The INGRES system, from the University of California, Berkeley, allows constraints to be speci ed with the database description Integrity constraints are provided as assertions and will be kept with the schema They do not a ect the structure of the database At execution time the assertion statements are merged, if relevant, with the queries or update statements and interpreted as if they were additional WHERE clauses Exploitation of the constraints to simplify queries may be done by a user who is sure that they have been applied throughout Some constraints applicable to Example 9-8 are shown in Example 9-11 For the COUNT functions introduced in Table 9-2 we use a token ( ALL) and type ( DISTINCT) notation which is symmetric and closer to INGRES Example 9-11
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