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Basic Structure of SQL Queries
• For any SQL as query, into and output both are relations.
• Number of relations inputs to a query will be at least one, but output will always be a single relation without any name unless specified.
• The basic structure of an SQL query consists of three clauses: select, from, and where. The query takes it’s input the relations listed in the from clause, operates on them as specified in the where and select clauses, and then produces a relation as the result without any name unless specified. A typical SQL query has the form
Select A1, A2,..., An (Column name)
from r1, r2,... , rm (Relation/table name)
Where P; (Condition)
• Each Ai represents an attribute, and each ri a relation, P is a predicate(condition). If the where clause is omitted, the predicate P is true.
• It is to be noted only select and from are mandatory clauses, and if not required then it is not essential to write where.
• SQL in general is not case sensitive i.e. it doesn’t matter whether we write query in upper or lower case.
• In the formal, mathematical definition of the relational model, a relation is a set. Thus, duplicate tuples would never appear in relations. In practice, duplicate elimination is time-consuming. Therefore, SQL allows duplicates in relations as well as in the results of SQL expressions. In those cases where we want to force the elimination of duplicates, we insert the keyword distinct after select, will discuss in detail later.
• SQL allows us to use the keyword all to specify explicitly that duplicates are not removed, Since duplicate retention is the default, we shall not use all in our examples
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