1. Chapter 4
An Introduction to SQL

2. SQL: History and Overview The Catalog Views Embedded SQL Cursors
Topics in this Chapter
SQL: History and Overview
The Catalog
Views
Embedded SQL
Cursors
Dynamic SQL
Call-Level Interface
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3. Early prototype called System R Current version is SQL:1999
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
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4. It is the standard language for relational systems, although imperfect
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
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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
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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
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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
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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
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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.
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10. Every database has a catalog A catalog has many schemas, one per user
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
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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)
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12. CREATE VIEW GOOD_SUPPLIER AS SELECT S#, STATUS, CITY FROM S
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’;
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13. SELECT GOOD_SUPPLIER .S#, GOOD_SUPPLIER .STATUS
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’;
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14. SQL statements can be embedded in a host program
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
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15. SQL statements after EXEC SQL can reference host variables
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
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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#);
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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
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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
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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’;
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20. A cursor is a buffer holding a row set
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
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21. You may then FETCH the cursor into variables you have declared
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
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22. Embedded SQL can be constructed and invoked dynamically
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;
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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
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