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.sql Enter 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.sql Enter 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 8

myusers J:\>mysql -h csweb.hh.nku.edu -u frank -p Enter 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 10

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 11

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, " "); 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 13

myusers INSERT INTO myusers VALUES (NULL, ‘Charles', ‘Frank', 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_list FROM tablename; – Column_list represents one or more attributes from tablename, separated by commas SELECT 15

E xample: SELECT name, city FROM customers;  the result contains all the rows and only the two specified columns of table customers. SELECT 16

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_list FROM tablename [ WHERE condition_list ] ; FWill retrieve all the rows that match the conditions specified in the optional WHERE clause FIf 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 value expression comparison_operator expression expression – 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 specified 19 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_list FROM 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 author 21 Listing Table Rows default

myusers mysql> select * from myusers; mysql> select from myusers where last_name='Frank'; mysql> select * from myusers where like '%edu';

Use the UPDATE command to modify data in a table. UPDATE command’s syntax: UPDATE tablename SET 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 23

Examples: UPDATE books SET price = price * 1.1; – increase all the book prices by 10% UPDATE customers SET address = ‘250 Olsens Road’ WHEREcustomer_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) 24

myusers mysql> select * from myusers; mysql> UPDATE myusers SET 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 26

Examples: DELETE FROMcustomers WHEREcustomer_id = 4; – delete data about a customer who didn’t place orders for a long time DELETE FROM books WHEREprice < 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) 27

myusers mysql> DELETE FROM myusers WHERE user_id = 8 LIMIT 1; mysql> select * from users limit 10;