SQL and JDBC

Peter Mork

Database Primer*

All data values are simple No arrays, pointers, vectors, complex types

All data are stored as 2D tables/relations Contains 0 or more rows/tuples Contains 1 or more columns/attributes

All operations defined logically Order of tuples is irrelevant

Keys used to identify unique tuples* SQL Instant Reference by Martin Gruber

SQL: What is it?

Data Definition Language (DDL) CREATE relations, attributes, etc.

Data Manipulation Language (DML) INSERT, UPDATE or DELETE data

Data Control Language (DCL) GRANT, REVOKE privileges

Data Query Language (DQL) SELECT data from relations

CREATE-ing Relations

CREATE TABLE {name}

( {col1} {type1} [{cons1}],

{col2} {type2} [{cons2}],

...

);

col? = A name for the columntype? = The column’s data-typecons? = An optional constraint on the column

Data types

CHAR(len), VARCHAR(len): Strings of maximum length len

FLOAT, REAL: Approximate numbersINT, INTEGER: Exact integersDECIMAL, DEC: Exact decimalsDATE, TIME, TIMESTAMP:

Timestamp combines date and time

Constraints

NOT NULL: No missing data allowedUNIQUE: Every value is unique or

missingPRIMARY KEY: Every value is uniquePlus other more sophisticated

predicates

Example

CREATE TABLE students

( s_id INT PRIMARY KEY,

s_name VARCHAR(50) NOT NULL,

s_dob DATE

);

s_id s_name s_dob

INSERT-ing New Values

INSERT INTO {table}

[ ( {col-a}, {col-b}, … ) ]

VALUES ( {val-a}, {val-b}, … );

col-x = Optional column namesval-x = A value for that columnIf no column names are given, the order

in the CREATE statement is used.

Example

INSERT INTO students

VALUES ( 001, ‘Peter Mork’ );Since no column names were specified:

001 is s_id ‘Peter Mork’ is s_name

NULL is used for missing data

s_id s_name s_dob

1 Peter Mork NULL

DELETE-ing Values

DELETE FROM {table}

[ WHERE {predicate} ];Deletes all tuples from {table} that

match {predicate}Use a primary key to isolate one tupleExample:DELETE FROM students

WHERE s_id = 1;

SELECT-ing Results

SELECT {attr-list}

FROM {table-list}

[ WHERE {pred-list} ];

Logically: Computes cross-product of all tables Discards results that don’t match

predicates Returns listed attributes

Simple Example

SELECT s_name

FROM students

WHERE s_dob > ‘1975-1-1’;

This retrieves all students born since 1975.

SELECT Clause

An attribute list is either: * (indicating all columns) A list of unique attribute names:

Usually an attribute name will sufficeSometimes you need {table}.{attr}Can rename attributes using AS

Example:SELECT students.s_id AS id,

s_name, grades.grade

FROM Clause

A table list is a list of unique table names: Usually a table name will suffice Multiple occurrences of the same table

must be renamed using AS

Example:FROM students, final_grades AS grades

WHERE Clause

The predicate list contains Join predicates, which relate two

columns from different tables Select predicates, which relate a column

to a constant or another column in the same table

Example:WHERE grade >= 3.0

AND students.s_id = grades.s_id;

Full Example

SELECT students.s_id AS id,

s_name, grades.grade

FROM students,

final_grades as grades

WHERE grade >= 3.0

AND students.s_id =

grades.s_id;

Sample Data

s_id s_name s_dob

1 Alice NULL

2 Bob 1974-8-28

3 Cindy 1973-10-19

s_id course

grade

1 544 3.5

1 521 3.8

1 531 3.9

2 544 3.6

2 551 2.9

Cross-Product Results

students.s_id s_name s_dob grades.s_id course grade

1 Alice NULL 1 544 3.5

1 Alice NULL 1 521 3.8

1 Alice NULL 1 531 3.9

1 Alice NULL 2 544 3.6

1 Alice NULL 2 551 2.9

2 Bob 1974-8-28 1 544 3.5

2 Bob 1974-8-28 1 521 3.8

2 Bob 1974-8-28 1 531 3.9

2 Bob 1974-8-28 2 544 3.6

2 Bob 1974-8-28 2 551 2.9

3 Cindy 1973-10-19 1 544 3.5

3 Cindy 1973-10-19 1 521 3.8

3 Cindy 1973-10-19 1 531 3.9

3 Cindy 1973-10-19 2 544 3.6

3 Cindy 1973-10-19 2 551 2.9

Check Predicates

students.s_id s_name s_dob grades.s_id course grade

1 Alice NULL 1 544 3.5

1 Alice NULL 1 521 3.8

1 Alice NULL 1 531 3.9

1 Alice NULL 2 544 3.6

1 Alice NULL 2 551 2.9

2 Bob 1974-8-28 1 544 3.5

2 Bob 1974-8-28 1 521 3.8

2 Bob 1974-8-28 1 531 3.9

2 Bob 1974-8-28 2 544 3.6

2 Bob 1974-8-28 2 551 2.9

3 Cindy 1973-10-19 1 544 3.5

3 Cindy 1973-10-19 1 521 3.8

3 Cindy 1973-10-19 1 531 3.9

3 Cindy 1973-10-19 2 544 3.6

3 Cindy 1973-10-19 2 551 2.9

Final Result

id s_name Grade

1 Alice 3.5

1 Alice 3.8

1 Alice 3.9

2 Bob 3.6

Note

This is the logical order of operations. The database system will not choose such a brain-damaged approach.

Application developers/users do not need to know how to execute the query efficiently; access plans are chosen automatically by the database system.

Other capabilities

Grouping and aggregation Uses GROUP BY keyword Aggregation functions include:

COUNTSUMAVG

More sophisticated predicatesNested queries

JDBC: What is it?

API for database programsCollection of interfaces, and a few

key classesRelies on vendor-supplied drivers

(i.e., implementations of the interfaces)

Connectivity Protocols

JDBCDriver registered

with systemDownloaded (linked

at run-time by VM)Written in Javacan be linked to an

existing ODBC driver

ODBCDriver registered

with systemInstalled on host

machineWritten in Cde facto standard

JDBC Classes

Date, Time, Timestamp, Types Represent standard RDB types Mapped to Java type system

DriverManager/DriverPropertyInfo Used to initialize driver Analogous to the System class

JDBC Interfaces

Driver/Connection Used to communicate with database

Statement (Callable, Prepared) Used to package SQL

ResultSet Used to iterate through query result (cursor)

DatabaseMetadata/ResultSetMetaData Contains data about the data

Steps to manipulate DB

1. Load driver2. Connect to database3. Manipulate data4. Close database

1. Load driver

Explicitly:Class.forName(“driver name”)This creates a new instance of the

driverImplicitly:Update the Java system settings(See Java docs for more info)

2. Connect to database

getConnection(db)getConnection(db, uid, pwd)getConnection(db, info)

db = “jdbc:odbc:data-source-name”db = “jdbc:???://host:port/dbname”

Connection notes

Properties is a sub-class of HashTable

Used to package multiple parameters

close() closes a connection (step 4)isClosed() tests a connection’s status

3. Manipulate data

createStatement establishes a framework for executing queries

executeQuery returns a ResultSetexecuteUpdate returns an intexecute can return either, but is

usually used when there is no data to return

Which execute to execute?

DDL and DCL queries are generally run using execute()

DML queries are generally run using executeUpdate(); the return value represents the number of rows affected

DQL queries are generally run using executeQuery(); a collection of tuples is returned

ResultSet

A cursor that iterates through a collection of tuples

Forward only!Each Statement object can have at

most one active ResultSet

Metadata

Metadata lets you know what the database looks like

Information about the table names, field names, domains, etc.

Metadata exists for both the Database (effectively constant) and for each ResultSet (depends on the query)

Important Metadata methods

columnCount(): The number of fields in a ResultSet

columnType(i): The type (as listed in Types) of column i. Note that columns are 1-indexed, not 0.

The return value of columnType can be used to select the correct getXXX method to invoke on the ResultSet

Possible Uses of JDBC

Leverage Java’s GUI tools to allow users to visualize the contents of a database

Use Java to publish information stored in a database on the web

Create tools to aid the database programmer in designing queries

Observations

Java’s inherent speed problems can be offset by the power of an efficient database

Many databases have awkward user interfaces

Database queries are disk/network requests -- think multi-threading