SQL By: Toan Nguyen

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Basic Structure of SQL Select List the attributes that you want to used in the result of the query. From Where you look for the relations to be evaluation. Where Consist of predicate involving attributes of the relations that appear in the from clause

Projection in SQL To eliminate some of the components of the chosen tuples, we can project the relation produced by an SQL query onto some of its attributes. In place of the * of the SELECT clause, we may list some of the attributes of the relation mentioned in the FROM clause.

Example Movie (title, year, length, inColor, studioName, producerC#) Select title, length From Movie Where studioName = ‘Disney’ AND year = 1990 title length

Selection in SQL The selection operator of relational algebra, and much more, is available through the WHERE clause of SQL. We may build expressions by comparing values using the six common comparison operators: =, <>,, =

Example SELECT title FROM Movie WHERE (year>1970 OR length<90) AND studioName = ‘MGM’;

SQL Queries and Relational Algebra SELECT L FROM R WHERE C

Dates and Times These values are often represent in a variety of format such as 5/14/1948 or 14 May 1948 Example: DATE ‘ ’ TIME ’15:00:02.5’

Ordering the Output We may ask the tuples produced by a query be presented in sorted order. ORDER BY Uses keywords DESC for descending and ASC for ascending. Example SELECT * FROM Movie WHERE studioName = ‘Disney’ AND year = 1990 ORDER BY length, title;

Subqueries In SQL, a query that is part of another query is call subquery. Subqueries can have subqueries, and so on, down as many level as we wish. There are a number of other ways that subqueries can be used:

Subqueries can return a single constant, and this constant can be compared with another value in a WHERE clause Subqueries can return relations that can be used in various ways in WHERE clauses. Subqueries can have their relations appear in FROM clauses, just like any stored relation can.

Example SELECT name FROM MovieExec WHERE cert# = (SELECT producerC# FROM Movie WHER title = ‘Star Wars’ );

Unions, Intersections, and Differences Uses unions to find the two relations that are not the same between the two queries. Uses intersections to find the two relations that are the same in both queries. Uses differences to find the relation in one queries but not in the other.

Example Unions = A U B Intersections = A ח B = C Differences = A - B

Example Unions SELECT title, year FROM Movie UNION ALL SELECT movieTitle AS title, movieYear AS year FROM StarsIn; Intersections INTERSECT ALL Differences EXCEPT ALL

Aggregate Functions These functions take a collection of values as input and return a single value. SQL uses five aggregation operators: 1) Average: avg 2) Minimum: min 3) Maximum: max 4) Total: sum 5) Count: count

Null Values Null value is uses to indicate absence information about the value of an attribute. Null value is uses to represent the value that are unknown, value that are inapplicable, and value that are withheld.

References Course Textbook a First Course in Database System (Jeffrey D. Ullman and Jenifer Widom)