1. Chapter 9 SQL and RDBMS Part C. SQL
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2. Copyright 2005 Radian Publishing Co.
Contents
Chapter 9 SQL and RDBMS
9.1 What is SQL
9.1 A. DDL
9.1 B. DML
9.1 C. DCL
9.2 SQL and RDBMS
9.2 A. MS Access 97, 2003 and Visual FoxPro
9.2 B. MySQL
9.3 Creating a Database
9.4 Creating a Table
9.5 Creating a Table with Constraints
9.6 Filling Table with Data
9.7 Other DDL Commands
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Chapter 9 SQL and RDBMS
SQL stands for Structured Query Language.
A relational database management system (RDBMS) fully supports
SQL.
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9.1 What is SQL
Set-oriented means that SQL processes sets of data in groups.
Non-procedural means that SQL does not care about how the result is
obtained, but it stresses on what result will be obtained.
SQL commands can be classified into three types:
Data Definition Language (DDL) commands
Data Manipulation Language (DML) commands
Data Control Language (DCL) commands
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9.1 A. DDL
Data Definition Language (DDL) commands are used to create, alter
and delete tables, views and indexes, including
CREATE DATABASE
DROP DATABASE
CREATE TABLE
DROP TABLE
ALTER TABLE
CREATE INDEX
DROP INDEX
CREATE VIEW
DROP VIEW
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9.1 B. DML (1/2)
Data Manipulation Language (DML) commands are used to update,
insert, modify and query the data in the database, including
SELECT
INSERT
UPDATE
DELETE
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9.1 B. DML (2/2)
The reasons for writing programs to issue SQL commands are
meeting specific needs of an organisation
providing interface for users who do not know SQL
handling errors
enforce database security
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9.1 C. DCL
Data Control Language (DCL) commands help DBA to control and
maintain the database, including GRANT, ALTER USER etc.
DBA stands for database Administrator (see Chapter 3)
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9.2 SQL and RDBMS
The SQL standards are promoted by two standards organisations: the
American National Standards Institute (ANSI) and International
Standards Organisation (ISO). In the textbook, ANSI-92 (launched in
1992) will be used.
Proprietary Extensions mean including extension to the standards, like
ANSI-92.
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10. 9.2 A. MS Access 97, 2003 and Visual FoxPro
MS Access® 97, MS Access® 2003 and MS Visual FoxPro® 5.0 are
software products of Microsoft.
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9.2 B. MySQL
MySQL is an open source RDBMS product, meaning that it is possible
for anyone to use and modify the software.
MySQL always works in the client-server mode, even in a standalone
computer.
Fig.9.1 Versions of MySQL
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12. 9.3 Creating a Database (1/2)
Fig.9.3 Handling of database files by different DBMS
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13. 9.3 Creating a Database (2/2)
In MS Access, related tables are stored in a single file.
In Visual FoxPro, database container is optional.
The SQL command to create database:
CREATE DATABASE dBName;
MySQL command to show database:
SHOW DATABASES;
MySQL command to open a database:
USE dBName;
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9.4 Creating a Table (1/3)
The basic syntax for creating a new table is
CREATE TABLE TableName1
(FieldName1 FieldType [(FieldWidth [, Precision])] [NOT NULL]
[PRIMARY KEY] [UNIQUE]
[, FieldName2 ...]);
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9.4 Creating a Table (2/3)
Fig.9.4 Table to be created
Fig.9.5 Structure of the
table to be created
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9.4 Creating a Table (3/3)
The SQL statement for creating the table BRASSITEM
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17. 9.5 Creating a Table with Constraints (1/2)
Some DBMS enforces constraints to insure data integrity. The constraints
include
the field value is unique (keywords UNIQUE or PRIMARY KEY).
the field value is non-empty (keywords NOT NULL or PRIMARY KEY).
providing default field value (keyword DEFAULT)
generating a number for a numeric field when new record is created (keyword AUTO_INCREMENT)
make sure that a foreign key will be updated if the primary key is updated (keyword FOREIGN KEY, CHECK, CASCADE, RESTRICT, ON DELETE and ON UPDATE)
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18. 9.5 Creating a Table with Constraints (2/2)
MySQL command to show the structure of the table:
SHOW FIELDS FROM TableName;
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19. 9.6 Filling Table with Data (1/3)
The basic syntax for inserting a new record is:
INSERT INTO TableName [(FieldName1 [, FieldName2 ...])]
VALUES (Expression1 [, Expression2 ...]);
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20. 9.6 Filling Table with Data (2/3)
If all the fields will be filled with values, you may skip entering the field
names in the Insert statement.
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21. 9.6 Filling Table with Data (3/3)
The date format in ANSI-92 is YYYY-MM-DD. Dates should be
enclosed in quotation marks.
For example ' ' represents the 1st day in April of 1991.
This format applies to MySQL, MS Access 97 and MS Access 2003,
but not in Visual FoxPro.
In Visual FoxPro, however, the default date format is MM-DD-YYYY,
and braces are used instead of quotation marks.
For example { }
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9.7 Other DDL Commands (1/5)
The basic syntax for creating an index is:
CREATE INDEX IndexName
ON TableName (FieldName [ASC|DESC], ...);
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9.7 Other DDL Commands (2/5)
The syntax for changing the structure of a table is:
ALTER TABLE TableName1
ADD | ALTER (MODIFY|CHANGE) | DROP [COLUMN] FieldName1
FieldType [(FieldWidth [, Precision])];
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9.7 Other DDL Commands (3/5)
In MS Access and FoxPro, ALTER COLUMN is used to change the
size and data type of a field. But, you cannot change a field name.
In MySQL, MODIFY COLUMN is used to change the size and data type
of a field. CHANGE COLUMN is used to change a field name.
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9.7 Other DDL Commands (4/5)
The SQL command to delete a table:
DROP TABLE TableName;
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9.7 Other DDL Commands (5/5)
The SQL command to delete database:
DROP DATABASE DBName;
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27. Chapter 10 Queries and Updating
Part C. SQL
Chapter 10 Queries and Updating
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Contents
Chapter 10 Queries and Updating
10.1 Simple Query
10.1 A. Select *
10.1 B. Selecting Individual Columns
10.1 C. Queries with Distinction
10.1 D. Ordering the Result
10.1 E. Efficiency Issue with Order
10.1 F. Specifying Alias for Column
10.1 G. Simple Expressions in Query
10.2 The WHERE Clause
10.2 A. Using Logical Operators
10.2 B. The IN Operator
10.2 C. The BETWEEN … AND Operator
10.2 D. The LIKE Operator
10.3 Exporting the Query Results
10.3 A. Outputting as a New Table
10.3 B. Outputting as a a Text File
10.4 Updating and Deleting Records
10.4 A. The SQL UPDATE
10.4 B. The SQL DELETE
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29. Chapter 10 Queries and Updating
In this chapter, you will learn how to query the data contained in tables
using the SQL SELECT command. You will also learn how to update
and delete records using SQL UPDATE and DELETE commands.
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10.1 Simple Query
A query is a simple request to retrieve information from a database
using criteria.
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10.1 A. Select * (1/3)
In SELECT *, the asterisk is a wildcard character that tells DBMS to
return all the fields.
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10.1 A. Select * (2/3)
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10.1 A. Select * (3/3)
A cursor is the result of a query. It is a read-only table available for
browsing, reporting or input to application programs. It is temporary as
it will be removed when the application is closed.
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34. 10.1 B. Selecting Individual Columns
The SQL command allows you to display any columns you want.
SELECT FieldNames
FROM TableName;
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35. 10.1 C. Queries with Distinction
The keyword DISTINCT is used to remove duplicate records in the
query result.
SELECT DISTINCT FieldNames
FROM TableName;
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36. 10.1 D. Ordering the Result (1/2)
The basic syntax for ordering the result is
SELECT [DISTINCT] FieldNames
FROM TableName
ORDER BY FieldName1, FieldName2 ¡K [ASC|DESC]
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37. 10.1 D. Ordering the Result (2/2)
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38. 10.1 E. Efficiency Issue with Order
The efficiency of ordering can be improved by creating indexes.
If a certain ordering is frequently needed, an index should be created
for this ordering.
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39. 10.1 F. Specifying Alias for Column (1/2)
A column alias can provide more descriptive names for columns.
For MySQL and Visual FoxPro, alias can be used in references in the
ORDER BY and GROUP BY clauses, but this is not allowed in MS
Access.
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40. 10.1 F. Specifying Alias for Column (2/2)
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41. 10.1 G. Simple Expressions in Query
A query may consist of expressions instead of field names in the
SELECT clause.
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10.2 The WHERE Clause
The syntax of a SELECT statement with a WHERE clause is
SELECT [DISTINCT] SelectItems
FROM TableName
WHERE Conditions
ORDER BY OrderItem1 [, OrderItem2 ] ;
Alias is not allowed in the WHERE clause for all DBMS.
An exact match is required for both MySQL and MS Access, but not for
Visual FoxPro. (Note: All the DBMS mentioned in the textbook are not
case-sensitive. )
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43. 10.2 A. Using Logical Operators
Logical operators, AND, OR and NOT, are used in the WHERE clause.
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10.2 B. The IN Operator
The IN operator returns true if the specified data matches any one of
the elements in the given set.
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45. 10.2 C. The BETWEEN … AND Operator
The BETWEEN … AND operator returns true if the specified data falls
between a starting value and an ending value inclusively.
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10.2 D. The LIKE Operator (1/2)
The LIKE operator compares the skeletons of two strings.
The wildcard (_) means any single character in the position.
The wildcard (%) means any string in forming the skeleton.
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10.2 D. The LIKE Operator (2/2)
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48. 10.3 Exporting the Query Results
Since the result of a query will be lost when the application is closed,
sometimes, you might want to store the results.
Fig.10.2 Variations between DBMS in exporting query results
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49. 10.3 A. Outputting as a New Table
In MS Access, the syntax to output the query as a table is:
SELECT [DISTINCT] SelectItems
INTO OutTable
FROM TableName ...
In MySQL, the syntax to output the query as a table is:
CREATE TABLE OutTable
SELECT SelectItems
In Visual FoxPro, the syntax to output the query as a table is:
INTO TABLE OutTable
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50. 10.3 B. Outputting as a a Text File
In MySQL, the syntax to output the query result to a text file is:
SELECT SelectItems
INTO OUTFILE 'FileName'
FROM TableName ...
In Visual FoxPro, the syntax to output the query result to a text file is:
TO FILE 'FileName'
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51. Copyright 2005 Radian Publishing Co.
10.4 A. The SQL UPDATE (1/2)
The basic syntax for updating a set of records is
UPDATE TableName
SET FieldName1 = Expression1 [, FieldName2 = Expression2 ]
WHERE Conditions;
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10.4 A. The SQL UPDATE (2/2)
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10.4 B. The SQL DELETE (1/2)
The basic syntax for deleting a set of records is
DELETE FROM TableName
WHERE Conditions;
If the WHERE clause is missed, all records will be deleted.
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10.4 B. The SQL DELETE (2/2)
Delete those records with quantity less than 5
Delete all records
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55. Chapter 11 Functions and Groups
Part C. SQL
Chapter 11 Functions and Groups
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56. Copyright 2005 Radian Publishing Co.
Contents
Chapter 11 Functions and Groups
11.1 Using Functions in Queries
11.1 A. String Functions
11.1 B. Numeric Functions
11.1 C. Date Functions
11.1 D. Logical Functions
11.2 Aggregate Functions
11.2 A. The Function COUNT()
11.2 B. The Function SUM()
11.2 C. The Function AVG()
11.2 D. The Function MAX()
11.2 E. The Function MIN()
11.3 Group
11.3 A. Group and Aggregate Functions
11.3 B. Grouping by Multiple Columns
11.3 C. Applying Filtering before Grouping
11.3 D. The HAVING clause
11.3 E. Combing HAVING and WHERE
11.3 F. Grouping by Non-column Names
11.3 G. Using GROUP BY with ORDER BY
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57. Chapter 11 Functions and Groups
We shall make use of the functions available in your DBMS to give
more varied results from a database.
A function accepts one or more input data through the
arguments and returns a result (see textbook Appendix D).
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58. 11.1 Using Functions in Queries
The built-in functions available from DBMS are categorised into four
main classes:
String
Numeric
Date
Logical
(see Appendix D on the Textbook for more information)
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11.1 A. String Functions
Common string functions include:
LENGTH(String)
UPPER(String), LOWER(String)
LTRIM(String), RTRIM(String), TRIM(String),
LEFT(String, n), RIGHT(String, n), MID(String, n, m),
INSTR(String, SubStr)
ASCII(String)
CHAR(String)
CONCAT(String1, String2, ...), etc.
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11.1 B. Numeric Functions
Common numeric functions include:
ABS(X)
SQRT(X)
ROUND(X,D), TRUNCATE(X, 0)
MOD(N,M)
POW(X,Y)
RAND()
FORMAT(X, D), etc.
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11.1 C. Date Functions
Common date functions include:
CURDATE()
DAY(Date), MONTH(Date), YEAR(Date)
DAYOFWEEK(Date)
DATE_ADD(Date)
DAYNAME(Date), MONTHNAME(Date), etc.
MS Access does not allow reserved words, like DAY, MONTH and YEAR, as column aliases.
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62. 11.1 D. Logical Functions (1/3)
The logical function is:
IF(expr1, expr2, expr3).
If expr1 is TRUE then IF() returns expr2, else it returns expr3.
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63. 11.1 D. Logical Functions (2/3)
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64. 11.1 D. Logical Functions (3/3)
Note
MS Access does not accept alias in the ORDER BY clause.
Therefore, the expression IIF(…) has to be repeated.
For Visual FoxPro, alias must be used in the ORDER BY clause
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65. Copyright 2005 Radian Publishing Co.
11.2 Aggregate Functions
Aggregate functions return a value based on multiple values in a
column. Examples are:
COUNT()
SUM()
AVG()
MAX()
MIN()
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66. 11.2 A. The Function COUNT() (1/2)
COUNT() returns the number of rows retrieved by a SELECT
statement. Usually, it is used as COUNT(*).
It does not make sense to mix an aggregate function with unrelated
fields.
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67. 11.2 A. The Function COUNT() (2/2)
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11.2 B. The Function SUM()
SUM() adds the specified fields in a column and returns the sum.
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11.2 C. The Function AVG()
AVG() function computes the average of a column.
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11.2 D. The Function MAX()
MAX() returns the largest value in a column.
The MAX() function can also work on dates and strings.
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11.2 E. The Function MIN()
MIN() works returns the lowest number of a column.
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11.3 Group (1/2)
Records which have the same value in a column can form a group.
The syntax of SELECT is now extended to:
SELECT [DISTINCT] SelectItems
FROM TableName
WHERE Conditions
GROUP BY GroupColumn1 [,GroupColumn2 ]
ORDER BY OrderItem1 [, OrderItem2 ... ;
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11.3 Group (2/2)
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74. 11.3 A. Group and Aggregate Functions
The power of GROUP BY is evident when it is used with aggregate
functions. The aggregate function returns a value for each group.
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75. 11.3 B. Grouping by Multiple Columns
Similar to ORDER BY, you may put more than one field in the GROUP
BY clause.
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76. 11.3 C. Applying Filtering before Grouping
Filtering of records is done by the WHERE clause.
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77. Copyright 2005 Radian Publishing Co.
11.3 D. The HAVING clause (1/2)
HAVING clause is designed to work with aggregate functions or group.
The HAVING clause must be placed after GROUP BY and before
ORDER BY.
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11.3 D. The HAVING clause (2/2)
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79. 11.3 E. Combing HAVING and WHERE (1/2)
Don't use HAVING for items that should be in the WHERE clause.
WHERE clause filters records and HAVING clause filters groups.
In combing the two, the WHERE clause is placed before the HAVING
clause and will be performed first.
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80. 11.3 E. Combing HAVING and WHERE (2/2)
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81. 11.3 F. Grouping by Non-column Names (1/2)
Expressions can be used in the GROUP BY clause.
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82. 11.3 F. Grouping by Non-column Names (2/2)
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83. 11.3 G. Using GROUP BY with ORDER BY (1/2)
The order of execution is: WHERE, GROUP BY, HAVING, ORDER BY.
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84. 11.3 G. Using GROUP BY with ORDER BY (2/2)
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85. Chapter 12 Joining Tables
Part C. SQL
Chapter 12 Joining Tables
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86. Copyright 2005 Radian Publishing Co.
Contents
Chapter 12 Joining Tables
12.1 Business Applications
12.2 Set Operations
12.2 A. Union
12.2 B. Intersect
12.2 C. Minus
12.3 Other Types of JOIN
12.3 A. Cross-join
12.3 B. Outer-join
12.3 C. Simulating MINUS by Outer-Join
12.3 D. Joining to Oneself
12.3 E. Bus Routes Using Joining to Oneself
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87. Chapter 12 Joining Tables
Table joining gathers data from more than one table when retrieving
data from a database.
Join condition may be placed in JOIN clause. It can also be an
inequality.
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88. 12.1 Business Applications
The basic syntax of inner join is:
SELECT …
FROM TableName1, TableName2
WHERE JoinCondition
[AND FilterCondition]
The filter condition must be written after the join condition.
An alternative syntax for inner-join is:
SELECT ...
FROM TableName1
INNER JOIN TableName2
ON JoinCondition
WHERE FilterCondition
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89. Copyright 2005 Radian Publishing Co.
12.2 Set Operations (1/3)
A join combines tables by columns.
Fig.12.7 Joining tables by columns
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90. Copyright 2005 Radian Publishing Co.
12.2 Set Operations (2/3)
Combining tables by rows requires the original tables to have
compatible structures.
Fig.12.8 Joining tables by rows
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91. Copyright 2005 Radian Publishing Co.
12.2 Set Operations (3/3)
We shall discuss three mathematical set operations:
UNION
INTERSECT
MINUS
Fig.12.9 The three set operations
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12.2 A. Union (1/2)
UNION simply groups the records of two tables together. Duplicated
rows are removed. If duplicated rows are necessary, use UNION ALL.
Syntax for UNION:
SELECT … FROM TableA
UNION
SELECT … FROM TableB
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12.2 A. Union (2/2)
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12.2 B. Intersect (1/2)
INTERSECT selects records which are common to both tables.
Syntax for INTERSECT:
SELECT … FROM TableA
INTERSECT
SELECT … FROM TableB
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12.2 B. Intersect (2/2)
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12.2 C. Minus
MINUS selects records which are found in one set but not on the other.
Syntax for MINUS:
SELECT … FROM TableA
MINUS
SELECT … FROM TableB
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12.3 Other Types of JOIN (1/2)
Fig All possible
table combination
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12.3 Other Types of JOIN (2/2)
A cross-join is a join without any join condition.
Joins with a join condition are divided into inner-joins and outer-joins.
An inner-join consists of records matching the join conditions. Inner-
joins are further classified into equi-joins and non-equi-joins.
An equi-join uses the equal operator (=) in the join condition.
A non-equi-join uses relational operators, >, <, >=, <=, <>.
A natural join is an equi-join without duplicating columns.
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99. Copyright 2005 Radian Publishing Co.
12.3 A. Cross-join (1/3)
A cross-join is a join without any join condition. It will map every record
in one table with every record in the other table.
Fig Cross-joining Table A and Table B
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12.3 A. Cross-join (2/3)
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12.3 A. Cross-join (3/3)
Qualifiers are used to remove ambiguity, where two tables have the
same column name.
Both join condition and filter condition can exist in the WHERE clause.
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12.3 B. Outer-join (1/6)
An outer-join selects both matched and non-matched records.
It is further classified into left-outer-joins, right-outer-joins and full-outer-
joins.
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12.3 B. Outer-join (2/6)
A left-outer-join selects all the rows from the first table including those
match and those do not match the rows in the second table.
The syntax for a left-outer-join is
SELECT ...
FROM TableName1
LEFT OUTER JOIN TableName2
ON JoinCondition
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12.3 B. Outer-join (3/6)
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12.3 B. Outer-join (4/6)
A right-outer-join selects all the rows from the second table including
those match and those do not match the rows in the first table.
Syntax for right-outer-join:
SELECT ...
FROM TableName1
RIGHT OUTER JOIN TableName2
ON JoinCondition
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12.3 B. Outer-join (5/6)
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12.3 B. Outer-join (6/6)
A full-outer-join reveals unmatched records on both sides as well as the matched records.
Syntax for full-outer-join:
SELECT ...
FROM TableName1
FULL OUTER JOIN TableName2
ON JoinCondition
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108. 12.3 C. Simulating MINUS by Outer-Join
Refer to textbook P.286
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109. 12.3 D. Joining to Oneself (1/2)
Joining a table to itself means writing the same table name twice in
FROM clause of the SQL statement.
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110. 12.3 D. Joining to Oneself (2/2)
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111. 12.3 E. Bus Routes Using Joining to Oneself
Refer to textbook P.289
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112. Chapter 13 Subqueries and Views
Part C. SQL
Chapter 13 Subqueries and Views
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113. Copyright 2005 Radian Publishing Co.
Contents
Chapter 13 Subqueries and Views
13.1 Introduction to Subqueries
13.2 Type 1: Single-valued Subqueries
13.2 A. Subqueries with Aggregate Functions
13.2 B. Involving two different tables
13.3 Type 2: Multi-valued Subqueries
13.3 A. The IN Operator
13.3 B. The = ANY Operator
13.3 C. The EXISTS Operator
13.4 Further Examples
13.5 Views
13.5 A. Creating a Simple view
13.5 B. Finding the Mode
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114. Chapter 13 Subqueries and Views
A subquery is a query which is embedded in another query.
A view is a subset of the database that is presented to some users.
It is a virtual table that does not take up as much physical space as an
ordinary table does, yet it allows you to perform query and updating to
the database.
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115. 13.1 Introduction to Subqueries (1/2)
If a query consists of a subquery, the subquery will always be executed
first. There are several variations in subqueries:
The subquery and the main query may refer to the same table, or different tables.
The result of a subquery may be a single value, or a set of values.
The subquery may be placed in the WHERE clause, or in the FROM clause.
A subquery may consist of another subquery, i.e. multi-sub-level.
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116. 13.1 Introduction to Subqueries (2/2)
Singled-valued subqueries need relational operators,
like =, >, >=, <>, <, <=;
Multi-valued subqueries need operators like IN, ANY and EXISTS.
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117. 13.2 Type 1: Single-valued Subqueries
Single-valued subquery:
SELECT ... FROM Table1
WHERE FieldName1 =|>|>=|<|<=|<>
(SELECT ... from Table2)
The query and the subquery may use the same table or different tables.
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118. 13.2 A. Subqueries with Aggregate Functions
A subquery with an aggregate function returns a single value.
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119. 13.2 B. Involving two different tables
The query and the subquery may use different tables.
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120. 13.3 Type 2: Multi-valued Subqueries
Some subqueries return a set of records. We cannot use simple
relational operators, like =, >, <, <> etc. Special operators, like IN and
EXISTS, are designed for multi-valued subqueries.
Multi-valued subquery:
SELECT ... FROM Table1
WHERE FieldName1 [IN | = ANY | EXISTS]
(SELECT ... from Table2)
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13.3 A. The IN Operator
The IN operator compares a piece of data with a set of values and
returns true if any one of the values in the set matches with the data.
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13.3 B. The = ANY Operator
The operator = ANY is the same as the IN operator.
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13.3 C. The EXISTS Operator
The function EXISTS() is a boolean function which tells whether the
result of a subquery is successful or not.
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13.4 Further Examples
Refer to textbook P. 314
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13.5 Views (1/2)
A view is a named query result from one or more tables in a database.
A view is often called a virtual table. You can perform queries and
updating on a view, yet it does not take up as much physical space as
an ordinary table.
When a view is changed, the tables that the view is built from will be
changed.
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13.5 Views (2/2)
The major advantages of views are:
A view can save the effort of entering the same complicated SQL statement every time.
A view can enforce security. It can prevent users from viewing other data that may be private or confidential.
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127. 13.5 A. Creating a Simple view (1/2)
The basic syntax creating a view is
CREATE VIEW ViewName AS SELECT ...
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128. 13.5 A. Creating a Simple view (2/2)
To make a query on the view SALESVIEW
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13.5 B. Finding the Mode (1/3)
We may use View to help finding the mode, which tells the most
popular quantity.
For example:
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13.5 B. Finding the Mode (2/3)
It is clear that “Central”, “Tai Kok Tsui” and “Tai Po” are popular district.
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13.5 B. Finding the Mode (3/3)
The results of the query show the most popular districts.
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