ILOG OPL Development Studio Language Reference Manual

Reading from a databaseIn OPL, database relations can be read into sets or arrays. For instance, these instructionsfrom the model file:tuple gasType {string name;float demand;float price;float octane;float lead;}tuple oilType {string name;float capacity;float price;float octane;float lead;}And these instructions from the data file:GasData from DBRead(db,"SELECT * FROM GasData");OilData from DBRead(db,"SELECT * FROM OilData");Together illustrate how to initialize a set of tuples from the relation OilData in the databasedb. In this example, the DBRead instruction inserts an element into the set for each tuple ofthe relations.Important conventions adopted by OPL:1. If read into a set, the resulting set must be a set of integers, floats, or strings, or a set oftuples whose elements are integers, floats, or strings.2. If read into an array, the resulting array must be an array of integers, floats, or strings, oran array of tuples whose elements are integers, floats, or strings.3. In the case of tuples, the columns of the SQL query result are mapped by position to thefield of the OPL tuples. For instance, in the above query, the column name has beenmapped to the field name and so on.4. When initializing an array with a DBRead statement, the indexing set and array cells areinitialized at the same time.Note: OPL does not parse the query; it simply sends the string to the database system thathas full responsibility for handling it. As a consequence, the syntax and the semanticsI L O G O P L D E V E L O P M E N T S T U D I O L A N G U A G ER E F E R E N C E M A N U A L59