《DatabaseSystemsISQLQueries》.ppt

Introduction We now introduce SQL, the standard query language for relational DBS. As RA, an SQL query takes one or two input tables and returns one output table. Any RA query can also be formulated in SQL, but in a more user-friendly manner. In addition, SQL contains certain features of great practical importance that go beyond the expressiveness of RA, e.g. sorting and aggregation functions. Example Instances We will use these instances of the Sailors and Reserves tables in our examples. Basic SQL Query relation-list: list of relation names (possibly with a tuple-variable after each name). target-list: list of attributes of relations in relation-list qualification : comparisons (“Attr op const” or “Attr1 op Attr2”, where op is one of ) combined using AND, OR and NOT. DISTINCT is an optional keyword indicating that the answer should not contain duplicates. Default is that duplicates are not eliminated! Conceptual Evaluation Strategy Semantics of an SQL query defined in terms of the following conceptual evaluation strategy: Compute the Cartesian product of relation-list. Selection of the tuples satisfying qualifications. Projection onto the attributes that are in target-list. If DISTINCT is specified, eliminate duplicate rows. This strategy is not an efficient way to process a query! An optimizer will find more efficient strategies to compute the same answers. It is often helpful to write an SQL query in the same order (FROM, WHERE, SELECT). Example Conceptual Evaluation Projection Projection Projection Selection Selection String Comparisons Null Values Null Values Truth Value Unknown Truth Value Unknown Ordering the Output Can order the output of a query with respect to any attribute or list of attributes. Add ORDER BY clause to the query: SELECT * FROM Sailors S WHERE age 20 ORDER BY rating; SELECT * FROM Sailors S WHERE age 20 ORDER BY rating, age; By default, ascending order. Use DESC t