Chapter 5

Introduction to SQL

• Structured Query Language • = the “programming language” for relational

databases• SQL is a nonprocedural language = the user

specifies what must be done (ex. create a table), but not how it is to be done.

• Several SQL dialects exist, in different RDBMSs; minor differences among them.

SQL

2

• Use CREATE TABLE command for each db table, basic syntax:CREATE TABLE tablename (

column_1 data_type [constraints] ,column_2 data_type [constraints],…column_k data_type [constraints] ,PRIMARY KEY (column_i [, column_j …]) ]

);

• Notes:– End each SQL command with a semicolon to execute it;– A comma separates all table element (column, PK) definitions;

SQL – Creating DB Tables

3

sql.sql

CREATE DATABASE sitename;

USE sitename;CREATE TABLE users (…First_name VARCHAR(20) NOT NULL,…};

VARCHAR

• Variable length string– Size varies from record to record.

create_myusers.sql

• CSC301\create_myusers.sql• How would you create the table in db_frank?

J:\>mysql -h cscdb.nku.edu -u frank -D db_frank -p < CSC301\create_myusers.sqlEnter password: ******

insert_myusers.sql

• CSC301\insert_myusers.sql• How would you upload this to

db_fall15_frank?

J:\>mysql -h csweb.hh.nku.edu -u frank -D db_fall15_frank -p < CSC301\insert_myusers.sqlEnter password: ******

• After you create the tables in your database, you can view:– All tables in the currently used database

with:

show tables;

– The structure of any of the tables in the

current database with:

describe table_name;

MySQL - SHOW and DESCRIBE

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myusers

J:\>mysql -h csweb.hh.nku.edu -u frank -pEnter password: ******mysql> use db_fall15_frank;mysql> show tables;mysql> describe myusers;

• Use DROP TABLE command for each db table you want to delete; basic syntax:

DROP TABLE tablename;

• Note:– This command deletes all the rows in the table tablename and the

table tablename itself!

SQL – Deleting DB Tables

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• SQL requires the use of the INSERT command to enter data into a table.

• INSERT command’s basic syntax:

INSERT INTO tablename

VALUES (value_1, value_2, … , value_n);

– This version of INSERT: • adds one table row (tuple) at a time• requires a value to be specified for every column of the table;

values are specified in the same order columns were defined in

DML – Adding Table Rows

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sql.sql

• INSERT INTO users …

• SHA1('mypass') – 160 bit hash digest of password

• NOW() – function return the current system date and time

• Example:insert into customers values (1, "Julie Smith", "25 Oak Street", "Airport West");insert into orders values (NULL, 1, 49.99, "2007-04-15");

• Notes:–The row contents are delimited by parentheses–Attribute entries are separated by commas–String and date values must be quoted (‘ or “)–Numerical entries are not enclosed in any special characters

INSERT

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myusers

INSERT INTO myusers VALUES (NULL, ‘Charles', ‘Frank', ‘frank@nku.edu', SHA1(‘secret'), NOW());

• SELECT command is used to list the contents of a table (or more tables).

• The simplest form (syntax) of a SELECT query is:

SELECT column_listFROM tablename;

– Column_list represents one or more attributes from tablename, separated by commas

SELECT

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• Example:SELECT name, city FROM customers; the result contains all the rows and only the two specified columns of table customers.

SELECT

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sql.sql

SELECT * FROM forums;

Asterisk (*) can be used as a wildcard character to list all attributes for the selected rows (when column_list is *)

SELECT user_id, username FROM users;

• Can limit data selected by placing conditional restrictions on the rows to be included in the output → with the WHERE clause of the SELECT statement.

• Syntax:SELECT column_listFROM tablename[ WHERE condition_list ] ;

Will retrieve all the rows that match the conditions specified in the optional WHERE clause

If no rows match the specified criteria result = an empty set of tuples (not an error!) 18

Listing Table Rows

condition_list = one or more conditional expressions connected by logical operators

• WHERE clause example:SELECT title FROM books WHERE price > 40;

• Conditional restrictions in the condition_list :column_name comparison_operator valueexpression comparison_operator expressionexpression – with columns and values (constants) as operands;

– Comparisons include the usual “suspects”• =, !=, <, etc.• Also can use BETWEEN value1 AND value2

– If a data field is empty, can test with IS NULL operator• SELECT name FROM customers WHERE address IS NULL;

Finds records with no address specified19

Listing Table Rows (cont)

• Can match patterns with LIKE– Pattern can be simple characters up to RE– % matches any number of characters, _ matches 1

character• Can match multiple conditions using AND and OR• Can negate a condition using NOT

• Example:SELECT title

FROM books WHERE price > 40 AND author like “Thomas%”;20

Listing Table Rows (cont)

• Can sort results of query with ORDER BY:SELECT column_listFROM tablename[ORDER BY column1 [ASC | DESC], column2 [ASC | DESC] …];

• Notes:–Although ORDER BY produces a sorted output, the actual table contents

are unaffected by the ORDER BY clause!–ORDER BY clause must be listed last in SELECT (except for LIMIT)

• Example:–SELECT title, price, author FROM books ORDER BY price, author; all books info sorted by price and, for same price, ordered by the author21

Listing Table Rows

default

myusers

mysql> select * from myusers;

mysql> select email from myusers where last_name='Frank';

mysql> select * from myusers where email like '%edu';

• Use the UPDATE command to modify data in a table.• UPDATE command’s syntax:

UPDATE tablenameSET column_1 = expression_1 [, column_2 = expression_2 …][WHERE condition_list];

–expression = a simple value (76 or ‘Florida’), or a formula (price – 10)–condition_list = one or more conditional expressions connected by logical operators (and, or, not)

• If more than one attribute is to be updated per tuple, separate modifications with commas

Updating Table Rows

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• Examples:UPDATE booksSET price = price * 1.1;– increase all the book prices by 10%

UPDATE customersSET address = ‘250 Olsens Road’WHERE customer_id = 4;• update a certain customer’s address – PK used to identify the customer

• Notes:– The WHERE clause is optional– If a WHERE clause is not specified, all rows from the specified table

will be modified.

Updating Table Rows (cont)

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myusers

mysql> select * from myusers;

mysql> UPDATE myusers SET email='mike@authors.com' WHERE user_id = 18;

mysql> select * from myusers where user_id='18';

• Use the DELETE command to delete data from a table.

• DELETE command’s syntax:

DELETE FROM tablename[WHERE condition_list ];

• Notes:– WHERE clause is optional– If a WHERE clause is not specified, all rows from the

specified table will be deleted

DML – Deleting Table Rows

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• Examples:

DELETE FROM customersWHERE customer_id = 4; – delete data about a customer who didn’t place orders for a long

time

DELETE FROM books WHERE price < 4;

• delete all cheap books (none, one, or more tuples can satisfy the condition)

DELETE FROM books;• delete all books; table is not deleted, but remains empty

Deleting Table Rows (cont)

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myusers

mysql> DELETE FROM myusers WHERE user_id = 8 LIMIT 1;

mysql> select * from users limit 10;