Relation operators
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scan (select-project) (one or more tables , a condition to satisfy, and an attribute list)
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sort (reorder)
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aggregate operators (SUM,AVG,MAX,MIN,...)
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insert/delete/update modification of tables
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set operators (union, intersection, difference)
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join, merge and division
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operators can be embedded recursively with the characteristics:
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uniformity of the data and operators as input and output is same data object (feature of 4GL)
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source of data-flow execution model
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