Chapter 4 An Introduction to SQL

Copyright © 2004 Pearson Addison-Wesley. All rights reserved.4-2 Topics in this Chapter SQL: History and Overview The Catalog Views Embedded SQL Cursors Dynamic SQL Call-Level Interface

Copyright © 2004 Pearson Addison-Wesley. All rights reserved.4-3 History of SQL Developed by IBM in 1970’s Early prototype called System R Current version is SQL:1999 All major database vendors support SQL All support a superset of a subset of SQL Both relvar and relation = table in SQL Tuple = row in SQL Attribute = column in SQL

Copyright © 2004 Pearson Addison-Wesley. All rights reserved.4-4 Overview of SQL It is the standard language for relational systems, although imperfect Supports data definition (DDL) and data manipulation (DML) DDL: CREATE TABLE, CREATE TYPE, ALTER TABLE DML: SELECT, INSERT, DELETE, UPDATE

Copyright © 2004 Pearson Addison-Wesley. All rights reserved.4-5 Relational operators in SQL Restrict is implemented by SELECT –Subset of rows –Uses WHERE clause to narrow result –Uses SELECT * to copy entire table Project is also implemented by SELECT –Subset of columns

Copyright © 2004 Pearson Addison-Wesley. All rights reserved.4-6 Relational operators in SQL Join is also implemented by SELECT –Result is a single table matched by values in the join columns of each source table –Uses WHERE clause to specify source tables –Uses dot operator to qualify like column names

Copyright © 2004 Pearson Addison-Wesley. All rights reserved.4-7 Update operators in SQL - Insert Insert is implemented by INSERT INTO –Ex,: INSERT INTO TEMP (P#, Weight) SELECT P#, Weight FROM P WHERE COLOR = COLOR (‘Red’); –Inserts part number and weight from P into part number and weight in TEMP for all red parts

Copyright © 2004 Pearson Addison-Wesley. All rights reserved.4-8 Update operators in SQL - Delete Delete is implemented by DELETE –Ex,: DELETE FROM SP WHERE P# = P# (‘P2’); –Deletes all shipments for part P2

Copyright © 2004 Pearson Addison-Wesley. All rights reserved.4-9 Update operators in SQL - Update Update is implemented by UPDATE –Ex,: UPDATE S SET STATUS = 2 * STATUS, CITY = ‘Rome’ WHERE CITY = ‘Paris’; –Doubles the status of the Parisian suppliers and moves them to Rome.

Copyright © 2004 Pearson Addison-Wesley. All rights reserved.4-10 The Catalog in SQL Every database has a catalog A catalog has many schemas, one per user Each catalog includes one Information Schema The Information Schema is a collection of all other schemas as views Represents a hypothetical Definition Schema for all users

Copyright © 2004 Pearson Addison-Wesley. All rights reserved.4-11 The Information Schema - Examples Tables (includes views) Views (does not include tables) Table constraints Referential constraints Column constraints Privileges (Because these objects are themselves tables, they can be queried)

Copyright © 2004 Pearson Addison-Wesley. All rights reserved.4-12 Views CREATE VIEW GOOD_SUPPLIER AS SELECT S#, STATUS, CITY FROM S WHERE STATUS > 15; You can now operate on the view SELECT S#, STATUS FROM GOOD_SUPPLIER WHERE CITY = ‘London’;

Copyright © 2004 Pearson Addison-Wesley. All rights reserved.4-13 Views - Example Creating a view, and then selecting from the view is equivalent to a select sub-query SELECT GOOD_SUPPLIER.S#, GOOD_SUPPLIER.STATUS FROM (SELECT S#, STATUS, CITY FROM S WHERE STATUS > 15 ) AS GOOD_SUPPLIER; WHERE GOOD_SUPPLIER.CITY = ‘London’;

Copyright © 2004 Pearson Addison-Wesley. All rights reserved.4-14 Embedded SQL SQL statements can be embedded in a host program C, COBOL, PL/1, Java, and many others Initiated in host via EXEC SQL statement Any SQL statement is valid after EXEC SQL

Copyright © 2004 Pearson Addison-Wesley. All rights reserved.4-15 Embedded SQL SQL statements after EXEC SQL can reference host variables Declare any such variables in a SQL DECLARE section Then, prefix : (colon) to the variable name when you reference it

Copyright © 2004 Pearson Addison-Wesley. All rights reserved.4-16 Embedded SQL - Example EXEC SQL BEGIN DECLARE SECTION; DCL SQLSTATE CHAR(5); DCL P# CHAR(6); DCL WEIGHT DECIMAL(5,1); EXEC SQL END DECLARE SECTION; P# = ‘P2’; EXEC SQL SELECT P.WEIGHT INTO :WEIGHT FROM P WHERE P.P# = (:P#);

Copyright © 2004 Pearson Addison-Wesley. All rights reserved.4-17 Embedded SQL - SQLSTATE IF SQLSTATE = ‘00000’ THEN … ELSE … Every host language must include a host variable called SQLSTATE that returns is execution is ok If no data found then it returns 02000

Copyright © 2004 Pearson Addison-Wesley. All rights reserved.4-18 Embedded SQL – Loosely coupled with the host Data types between host and tables must be compatible Host variables can have the same name as table columns, but need not SQL retrieval returns sets of rows Host languages handle one row at a time To allow this, SQL sets up a CURSOR to hold returned row sets, so they can be processed one row at a time

Copyright © 2004 Pearson Addison-Wesley. All rights reserved.4-19 Embedded SQL – Sometimes Cursors are not Needed SELECT, when it returns at most one row INSERT, however many rows DELETE, likewise UPDATE, ditto Example: EXEC SQL UPDATE S SET STATUS = STATUS + :RAISE WHERE CITY = ‘London’;

Copyright © 2004 Pearson Addison-Wesley. All rights reserved.4-20 Embedded SQL – Cursors A cursor is a buffer holding a row set The buffer is called the “context area” A cursor maintains a pointer to the currently active row, or record, and begins before, yes, before the first row You can process each row, one at a time, and then advance the cursor UPDATE and DELETE are supported when you reference the “CURRENT” row

Copyright © 2004 Pearson Addison-Wesley. All rights reserved.4-21 Embedded SQL –Cursors A cursor must be declared – here is where you place your SQL statement that will return a set Next OPEN the cursor You may then FETCH the cursor into variables you have declared Each FETCH reads a row into your variables –Often this is accomplished within a loop Your first FETCH reads the first row CLOSE when finished, please

Copyright © 2004 Pearson Addison-Wesley. All rights reserved.4-22 Dynamic SQL Embedded SQL can be constructed and invoked dynamically DCL SQLSOURCE CHAR VARYING (65000); SQLSOURCE = ‘DELETE FROM SP WHERE QTY < QTY (300)’; EXEC SQL PREPARE SQLPREPPED FROM :SQLSOURCE; EXEC SQL EXECUTE SQLPREPPED;

Copyright © 2004 Pearson Addison-Wesley. All rights reserved.4-23 SQL Call-Level Interface (CLI) Alternative to Embedded SQL Similar functionality, based in MS ODBC Better than Embedded SQL for two reasons –Can be compiled by the host compiler and distributed shrink wrapped to customers –Greater DBMS independence because they can interface with anyone’s SQL engine