Introduction to SQL

Structured Query Language (SQL)By

P.D. Krolak & M.S. KrolakCopyright 2001

SQL What is it? SQL is a natural language like, ANSI

standard for interfacing database systems.

Developed in the 70’s by IBM. SQL has been revised several times and

is supported by almost all commercial and/or widely used databases.

Designed to be similar to a declarative sentence and to be used by the non programmer.

SQL Statement SQL has a simple natural language

structure. Starts with a verb, followed by a

series of clauses and ends with “;”. Uses a small set of reserved words. SQL is used to communicate with

the relational database thru a standard API.

SQL Reserve Words Alter And As Create Cross Join Delete Drop From

Full Join Group by Insert Into Join Left Join Like Limit

SQL Reserve Words: (continued) On Or Order By Right Join Select Where

SQL has Two Parts Data Creation Language (DCL) to

create database objects. Data Manipulation Language (DML)

to manipulate the of data.

SQL SQL is case insensitive. SQL object names are case

sensitive. SQL objects name -- databases,

tables, columns, and passwords.

SQL SQL is used to:

create the database’s objects, to store and edit the data, to form queries, and to allow the DB system’s

administrator to: establish the users, define their privileges, and to provide maintenance.

SQL Data Creation Language

Basic Database Building Blocks

Create DatabaseCreate IndexCreate Table

SQL Database Creation Action -- Creates the database

object. Syntax –Create Database database_name ; Example:

Create Database ClassDatabase;

SQL Table Creation Syntax Syntax --Create Table table_name ( col1 type

restriction, col2 type restriction, … );

Each column name should be unique, start with a letter and be made up of letters, numbers, and “_”, and not use a reserved word.

Data types define what type of data object maybe stored in the column

Restrictions or constraints limit what kinds of information may be stored, e.g. unique or non null.

SQL Data Types There a wide variety of data types found

in SQL not all of whom are supported in the various commercial systems.

Common data types are integer, real, character, text, date and time, and binary (blob).

Not every DBM system supports every data type so read the manual.

MySQL supports the following data types

MySQL supports a wide variety of data types:

1. Integer (signed and unsigned) 1-8 byte2. Float (real) 4-8 byte3. Datetime4. Char (string) including fixed and

variable length, case sensitive, binary (blob)

5. Enumerated and set

MySQL supports the following integer data typesInteger Data Type Description

TinyInt(length) Unsigned Byte --1

Int1(length) Signed Byte --1

Smallint(length) Unsigned Byte --2

Int(length) Signed Byte --2

MediumInt(length) Unsigned Byte --3

Int3(length) Signed Byte --3

MiddleInt(length)

Int(length) Unsigned Byte --4

Integer(length) Signed Byte --4

Int4(length)

BigInt(length) Unsigned Byte --8

Int8(length) Signed Byte --8

MySQL supports the following numeric (float) data types

Numeric Data Type Description

Float Floating Point Number –4 Bytes

Float(length, decimal)

Float4(length, decimal)

DoublePrecision(length,decimal)

Double Precision Floating Point – 8 Bytes

Double(length,decimal)

Float8(length,decimal)

Foat8

MySQL supports the following DateTime data typesDateTime Data Type Description

Timestamp(Length) Timestamp updates with update, null stores current

Date Date value yyyy-mm-ddAssign null stores current

Time HH-MM-SS time

Datetime Stores date & time

Year Year in either YYYYY or YY format

MySQL supports the following Character data typesCharacter Data Type Description

Char(length) Fixed character size

Binary(length) Case Sensitive

VarChar(length) Varaible character, length maxium

VarBinary(length) Same as above and Case Sensitive

TinyIext Text up to 256 char

TinyBlob As above and Case Sensitive

Text(length) Text Field 64Kb

Blob A binary field up to 64Kb

MediumText Up to 16Mb

MediumBlob Up to 16MB

LongText Up to 4Gb

LongBlob Up to 4Gb

MySQL supports the following data types

Data Type Description

Enum An enumerated list of values

Set A set of predefined values that can be entered as comma separated list or as integer that represents the binary value for the value

Data Definitions

SQL Alter Action – To alter the definition of

the database objects. Syntax – Alter [Ignore] Table table_name

Specification [, Specification]

SQL Alter Specifications A few of the allowed specifications:

Add [Column] column_name (type, )[First or After column_name]

Add Index [index_name] (column_list) Add Primary Key (column_list) Add Unique [index_name] (column_list) Alter [Column] column_name {Set

Default default_value or Drop Default}

SQL Alter Specifications (cont.) More of the allowed specifications:

Drop [Column] column_name Drop Primary Key Drop Index index_name Modify [Column] create definition Rename new_name

SQL Alter Ignore The Ignore option causes rows with

duplicates in unique keys to be deleted. If no duplication, then no change.

SQL Describe Action – give the details of columns

in a selected table. Syntax –Describe table_name column_names

Describe Example

SQL Drop Action – Drops, i.e. deletes the

object. This is a non recoverable action so use cautiously.

Syntax – Drop Database [If Exists]

Database_name; Drop Table [If Exists] Table_name ;

SQL Explain Action – will display query for

select as in the show clause. Syntax –Explain Select_Query OR table_name

SQL Flush Action – Clears out the cache

memory Syntax –Flush flush option1 [, flush option2 ,

…];

SQL Flush options

Option Description

Hosts Clear only if IP address of host change

Logs Closes logs

Privileges Use after changes to the user privileges

Tables Close tables

Status Reset system status =0

SQL Show Action – Show the data in the

result set. Syntax –Show parameter;

Show Parameter The following parameters are used:

Columns From table_name Database_names Grants For user_names Index From table_names Process_list -- list running processes Status

Show Parameter (continued)

Tables From database_name Tables Status From database_name Variables – system variables

SQL Use Action – sets the active database. Syntax – Use database_name;Example –Use pizza_group3;

SQL Key Concepts

Keys are fundamental to creating a relational database.

SQL Primary Key Each row of a table will have a

primary key – a column or collection of columns that uniquely identifies the row. Thus each row will have a unique key. If two or more rows have the same key SQL will report an error.

SQL Multiple Column Keys

SQL Foreign Key A foreign key is a primary key in

another table. For instance, the Student Id number maybe the primary key in the table Student while it is a foreign key in the Class_roster.

SQL Index Each table’s rows need to be

unique, i.e. needs to have a unique primary key.

One method to achieve this is to give each row a row_id or numeric index. This index is auto-incremented every time a new row is inserted in the table.

SQL Data Manipulation Language (DML)

The DML allows the user to insert, query, and edit the data. The ability to form queries and to generate reports is the heart of any Information System

SQL Data Manipulation Insert records into the table. Update data within records. Delete records from tables. Select records from the table(s).

SQL Insert Action – Insert data into the table. Syntax –Insert [delayed|low priority] [into]

Table [(column, … ,)] Values (value, …,) ;

Example –Insert students (first, last, email, user-

id) values (clyde, smith, csmith@cs.uml.edu, csmith);

SQL Insert Delayed or Low Priority Options Delayed – delay action until table

is free. Will also bundle actions. Low Priority – delay action until the

table is not in use

SQL Update Action – Updates, changes, the selected

data without changing the definition of the table.

Syntax –Update table Set column=value,… [Where

Clause]; Example –Update Student Set user-id=cjsmith Where

first=clyde and last=smith;

SQL Update Set Clause Action – Set the column(s) to the

assigned value(s) Syntax –Set columnk =valuek

columnm=valuem, …

SQL Delete Action – deletes rows from table that

match the Where clause. Syntax –Delete From table [Where clause] ;If no Where clause, all the data is deleted

from the table. Example –Delete From Student Where user-

id=cjsmith;

Constructing the SQL Query

Select Statement1. Where Clause2. Group By Clause3. Having Clause4. Order By Clause

SQL Select Action – Select column(s) from the

table(s), if the row satisfies the clauses of the select. The row is said to be matched and is retained.

The resulting collection of rows is called the result set.

SQL Select Syntax – Select [All|Distinct] List_of_Columns

From List_of Tables [Where Clause] [Group By Clause] [Having Clause] [Order Clause]

SQL Select The option All specifies select all

the rows and is the default choice. The option Distinct specifies that

only those rows that contain unique values For the List_of_Columns will be selected, i.e. no duplicates

SQL Select List_of_Columns If the List_of_Columns, (column1, …), is

selected from more than one table, then they must be fully qualified. This is done by specifying the Table_name.Column_name. Note the dot between the table and the column.

* is used to specify all of the columns in the table.

SQL From List_of_Tables The From List_of_Tables, table1, …,

specifies the table(s) from which the column(s) will be taken.

When List_of_Tables involves more than one table, it is referred to as a Join.

SQL Where clause Action – Selects the rows based on

conditions or constraints formed through logical propositions.

An example of a condition is:Last=‘smith’The logic operators AND, OR, NOT

can also be used.First =“clyde” AND Last=“smith”

SQL Where clause (continued) An example of a compound logic

condition is:Using the logic operators AND, OR,

NOTFirst =“clyde” AND Last=“smith”Would select all records of students

named clyde smith

SQL Comparison Predicates

A comparison predicate is used by a Where clause to compare values. If the comparison is true the row is retained.

SQL Relational Operators

Symbol Operator

= Equal

<> or != Not Equal

< Less Than

> Greater Than

<= Less Than or Equal

>= Greater Than or Equal

SQL Like Retrieves rows if a substring is

located in a string. % represents an arbitrary string of

letters, numbers, and punctuation. The underscore, “_” matches a single

character.

SQL Like (continued) Syntax –Column Like ‘Substring%’ matches if the Substring starts the

String.Column Like ‘%Substring’ matches if the Substring ends the String.Column Like ‘%Substring%’ matches if the Substring is in the

String.

SQL Not Like Action – If the substring does not

match the contents of the column then the row is matched.

Syntax – Column Not Like ‘_substring’ Example –User_id NOT LIKE ‘_smith’

SQL IN IN is true if the column is in the list. Syntax –Value In (Value1, …) Example:Color IN (red, green, blue)If Color is red or green or blue then

return 1 else return 0

SQL Not IN Definition: Not IN is true if the

column is Not In the list. Syntax –Value Not In (Value1, …) Example:Color Not IN (red, green, blue)If Color is red or green or blue then

return 0 else return 1

SQL Between Function is true if the value is >=

lower_ value and <= upper_value. Syntax –value Between

(lower_value,upper_value)5 Between (2, 10) returns as true or

1

SQL IS Null Action – the predicate is matched if

the value is Null. Syntax –value IS Null

SQL IS Not Null Action – the predicate is matched if

the value is Not Null. Syntax –value IS Not Null

SQL All, Some, Any

SQL Exists Action – returns a true value if the

sub select result set returns at least on row.

Syntax --

SQL Unique Action – Similar to Exists it is used

only with a sub select. It is true only if the rows in the result set are unique.

Syntax --

SQL Overlaps Action – tests for time interval

overlaps to detect conflicts in schedules, etc.

Syntax –time_interval1 Overlaps time_interval2

Time intervals may be specified as: A start time + time interval A start time end time

SQL MATCH

Conditional Expressions

Conditional expressions are slightly more complicated expressions than those we have looked at previously.

SQL Case Expression It is similar to a Case statement found in

many programming languages but it is an expression and NOT a statement.

Action – A case expression takes a value and if it matches the when expression performs the then ..

If the result option is not defined the Case returns a Null if the Case is not matched.

SQL Case Expression syntax

SQL Case syntax (continued)

SQL Cast Expression Action – Syntax --

SQL Coalesce Function Action – Returns the first Non Null

value from an array. Syntax –Value Coalesce (value1, ….. )

SQL Select Example Queries

Select is a fundamental statement and deserves an

extensive set of examples to illustrate its many options.

SQL Select ExampleDisplay of the guest book’s visitors

Table output from MySQL & phpmyadmin

SQL Select – A more complex example:Select using Where, Order, Limit Clauses

Note: Where clause’s use of Like and Is Not Null

SQL Joins

Joins are fundamental to queries in relational DBMS. The location of data in unique cells forces the query to select data from multiple tables.

SQL Aliasing or the As Clause Action – allows for the substitution

of a short mnemonic for a longer database object name. Particularly useful in situations that

require the object be fully qualified such as joins.

SQL uses a dot notation to indicate how the object is contained in the database.

SQL Aliasing (continued) Syntax –Very.long.name As Short_name Example --Select s.first s.last gb.grade From

Student As s GradeBook As gb where s.student_id=gb.student_id;

SQL Aliasing (continued) Note the As is often dropped

database object in its long form is followed by a space and the short form

Using the previous example --Select s.first s.last gb.grade From

Student s GradeBook gb where s.student_id=gb.student_id;

SQL Cross Join Action – the Cross Join or Cartesian Join

takes every record of one table and joins it with every record of all the other table(s), i.e. the Cartesian product. This is a huge result set an is not

often used or useful. Syntax –Select List_of Columns From table_name

Cross Join List_of_Tables;

SQL Inner Joins Action – The join uses a Select from with

multiple tables and a where clause to filter the rows. The inner join produces the same outcome with a different syntax and uses the On clause instead of the Where clause.

Syntax –Select * From Table1 As T Inner Join Table2

As TT On (T.id =TT.T_id);

SQL On Clause used with Inner Join Action – the ON clause when used

in the Inner join is used in place of the Where Clause. It performs the same task of filtering

the rows of the Select. If the On Clause is true the Left Table

and the Right table are retained and rejected if False.

SQL On Clause used with Inner Join (continued) Syntax – Select List_of_Columns From

Left_table Inner Join Right_table On(Left_table_id = Right_table_id);

SQL Using Clause Action – Used in place of ON

Clause where the column names are the same in both tables and the values are equal. This is a short cut.

Syntax –Using (column_name1 …)

SQL Outer Joins Outer Joins are based on using two

tables the first is called the left and other is called the right. There are three types of outer joins:

1. Left Outer Join2. Right Outer Join3. Full Outer Join

SQL Left Outer Join Action – The Left_table is matched

to the Right_table. The Left and Right rows are retained

if there is a match. The Left row is retained if there is no

match and the Right row is Null filled.

SQL Left Outer Join Syntax Syntax --

SQL Right Outer Join Action – The Right_table is

matched to the Left_table. The Left and Right rows are retained

if there is a match. The Right row is retained if there is no

match and the Left row is Null filled. Symmetric to the Left Outer Join.

SQL Right Outer Join (NOTE)

Many RBDMS do not support the Right Outer Join since one has only to exchange the choice of Right and Left Table and perform the Left outer Join.

SQL Right Outer Join Syntax Syntax –Select List_columns From right_table Right Outer Join left_table

On[..] ;

SQL Full Outer Joins Action – The Full Outer Join performs

both a Left Outer Join and Right Outer Join. The filter on the match is to retain the

Left and Right row. For the unmatched rows:

The unmatched Left row is retained with the Right Row Null Filled.

Similarly for the unmatched Right row.

SQL Full Outer Joins Syntax –

SQL the On Clause is used with the Outer Joins The On Clause is used with the

Outer Joins. As in the Inner Join the ON acts like the Where Clause when the Right and Left Tables match.

SQL On Clause used with Inner Join (continued)ON Clause Used With .. Action When match is not

found does not follow Where Clause

Left Outer Join Left row retained and Right row is filed with NULLs.

Right Outer Join Right row retained and Left row is filed with NULLs.

Full Outer Join Both the Right and Left rows that are not matched are retained and the other row is Null filled.

SQL Join Remarks Since there can not be an Left, Right, or

Full Inner Join, it is permitted to drop the word and use Left Join, Right Join, or Full Join.

The Where Clause is used with the older form of the Join instead of the ANSI SQL92 form Inner Join.

ANSI SQL92 -- When using the Inner Join the use of of the Where Clause will generate an error.

SQL Union Joins

SQL Group By Clause Action – Takes all the rows that that

have data in specific column(s) and will allow aggregation functions to be preformed on them.

Syntax –Group By column_nameGroup By column_name_list

SQL Having Clause Action – Having is used to specify the

rules for choosing rows for the group. Chose those rows having satisfying the specified conditions. Having Clause follows Group By

Syntax –Having Specified_Conditions Example –Having grade_point >= 3.8

SQL Limit Action – the statement is applied

only to the first n occurrences (rows) that match. If there are two values specified the first, start <= n, results in the rejection of the first start occurrences.

Syntax –Limit (n)Limit (start, n)

SQL Order By Clause Action – is an option to display the

query results in ascending or descending order based on specified column(s).

Syntax –Order By Column_List [Asc |Desc]Here Asc is ascending order -- the

default and Desc is descending order.

SQL Functions

SQL supports a large collection of built in functions. It also allows user defined functions.

SQL Value Functions

1. String2. Numeric3. Datetime

SQL String Value Functions

SQL String Value FunctionFunction Description

Substring

Upper

Lower

Trim

Translate

Convert

SQL Numeric Value Functions

SQL Numeric Value FunctionsNumeric Function Description

Position

Character Length

Octet_length

Bit_Length

Extract

SQL Datetime Value Functions

SQL Datetime Value Functions

Datetime function Description

Current_Date

Current_time

Current_timestamp

SQL Expressions

SQL Value Expressions

1. String2. Numeric3. Datetime4. Interval5. Conditional

SQL Aggregate Functions

Aggregate functions are used to summarize the results of a query. The aggregate function is applied to either the full set of rows or a subset based on the Where Clause.

SQL Aggregation Functions:A partial list.Function Description: The operation

is applied to the column of the selected rows

Sum() Sums the numeric values of the column

AVG() Computes the average

MAX() Finds the maximum value

MIN() Finds the minimum.

COUNT() Counts the number of rows selected.

SQL An Example of the Aggregate Function Consider a sales manager who

wants the value of orders placed by each customer in the database.

Select customer_name.c sum(order_value.o) From customers as c and orders as o

where cust_id.c =cust_id.o Grouped By cust_id.o;

SQL User Defined Functions SQL allow the user to create their

own functions and to add them to the function library.

SQL Views

Views are database objects similar to tables but do not have an independent existence.

SQL Setting the Database’s Security

Database security is critical to the success of any serious application. The database administrator must manage the access to critical functionality and data.

SQL Privileges

SQL Grant Action – Establish the access rights

of the database user. Syntax –Grant privilege [column_list)]

[.privilege [column_list)] …] On {table} To user

[Identified By ‘password’] [, user Identified By ‘password’ …] [With Grant Option]

SQL Revoke Action – Revoke removes privileges

from a user using the Grant syntax. The Revoke must be done by some one

holding the Grant privilege to do so. Syntax –Revoke privilege [column_list)]

[.privilege [column_list)] …] On {table} To user

Advanced Issues:

Databases are not all identical in their capabilities. The following are a few features of SQL that are not supported by many DBMS and MySQL in particular.

SQL Transactions

Transactions allow the user to group two or more statements together.

SQL Transactions Not all Database systems support

the transaction concept. Action – The transaction group is

processed as a single thread with logic to allow the statements to “rollback the executed statements to the original state before the transaction.”

SQL Transaction Concept Think of transactions like updating a

column, taking an action on the update, and then closing the table.

What would happen if you phoned a friend, the phone rings but no one answers, you hang up but the charge log database did not get the message and continues to charge you.

SQL Transactions MySQL does not support

transaction as of now. A partial work around is possible

MySQL Transaction Work Around MySQL uses a Lock Table concept

to prevent corruption of the data. MySQL can not do a “Roll Back”

SQL Triggers

Triggers are a group of statements that are execute when a value or condition is true.

SQL Triggers MySQL does not support triggers!! There is no effective work around

using MySQL

SQL Subselects

Subselects are occur when the data you want to select requires a first step to determine and aggregate value or condition before the select can proceed.

SQL Subselect example Consider the query that a sales

manager might make – select customers who ordered more twice the average.

SQL below will NOT work due to lack of value Avg(order)

Select customer order from Customers where order> 2*Avg(order);

SQL Subselect Correct way to form the SQL Statement Select customer order Table

customers where order>2*(Select Avg(order) From order);

Here we first use the sub select to aggregate the orders and compute the average value. Then we perform the Select to find orders that exceed twice the average order value.

SQL Subselects Concept

SQL workarounds for RDBMS that don’t support subselects MySQL does not support

subselects. An effective work around --

Create a temporary table using the result set of the subselect step and then,

Perform the desired select against the temporary table.

SQL References Jager, R.J., Reese, G., King, T.,

“MySQL & MSQL”, O’Reilly Press (1999)

Maslakowski, M., Butcher, T., “Teach Yourself MySQL in 21 Days”, SAMS (2000)

Taylor, A. G., “SQL For Dummies”, IDG Books, 3rd Edition, (1998)