SQL Commands

Database

A database consists of one or more tables. A table is identified by its name. A table is made up of columns and rows. Columns contain the column name and data type. Rows contain the records or data for the columns.
Basic SQL

Each record has a unique identifier or primary key. SQL, which stands for Structured Query Language, is used to communicate with a database. Through SQL one can create and delete tables. Here are some commands:

CREATE TABLE – creates a new database table
ALTER TABLE – alters a database table
DROP TABLE – deletes a database table
CREATE INDEX – creates an index (search key)
DROP INDEX – deletes an index

SQL also has syntax to update, insert, and delete records.

SELECT – get data from a database table
UPDATE – change data in a database table
DELETE – remove data from a database table
INSERT INTO – insert new data in a database table

SELECT

The SELECT is used to query the database and retrieve selected data that match the specific criteria that you specify:

SELECT column1 [, column2, …]
FROM tablename
WHERE condition

The conditional clause can include these operators

= Equal
> Greater than
< Less than >= Greater than or equal
<= Less than or equal <> Not equal to
LIKE pattern matching operator

SELECT * FROM tablename

returns all the data from the table.

Use single quotes around text values (most database systems will also accept double quotes). Numerical values should not be enclosed in quotes.

LIKE matches a pattern. The wildcard % is used to denote 0 or more characters.

‘A%’ : matches all strings that start with A
‘%a’ : matches all strings that end with a
‘%a%’ : matches all strings that contain an a

CREATE TABLE

The CREATE TABLE statement is used to create a new table. The format is:

CREATE TABLE tablename
(column1 data type,
column2 data type,
column3 data type);

char(size): Fixed length character string.
varchar(size): Variable-length character string. Max size is specified in parenthesis.
number(size): Number value with a max number of columns specified in parenthesis
date: Date value
number(size,d): A number with a maximum number of digits of “size” and a maximum number of “d” digits to the right of the decimal

INSERT VALUES

Once a table has been created data can be inserted using INSERT INTO command.

INSERT INTO tablename
(col1, … , coln)
VALUES (val1, … , valn)
UPDATE

To change the data values in a pre existing table, the UPDATE command can be used.

UPDATE tablename
SET colX = valX [, colY = valY, …]
WHERE condition
DELETE

The DELETE command can be used to remove a record(s) from a table.

DELETE FROM tablename
WHERE condition

To delete all the records from a table without deleting the table do

DELETE * FROM tablename
DROP

To remove an entire table from the database use the DROP command.

DROP TABLE tablename
ORDER BY

ORDER BY clause can order column name in either ascending (ASC) or descending (DESC) order.

ORDER BY col_name ASC
AND / OR

AND and OR can join two or more conditions in a WHERE clause. AND will return data when all the conditions are true. OR will return data when any one of the conditions is true.
IN

IN operator is used when you know the exact value you want to return for at least one of the columns

SELECT * FROM table_name WHERE col_name IN (val1, val2, …)
BETWEEN / AND

The BETWEEN … AND operator selects a range of data between two values. These values can be numbers, text, or dates.

SELECT * FROM table_name WHERE col_name BETWEEN val1 AND val2
JOIN

There are times when we need to collate data from two or more tables. That is called a join. Tables in a database are related to each other through their keys. We can associate data in various tables without repeating them. For example we could have a table called Customers which could have information about customers like their name, address, phone numbers. We could have another table called Products that has information regarding the products like part number, product name, manufacturer, number in stock, unit price. A third table called Orders could have information regarding what product was ordered, by whom, the date the order was placed, and quantity. Here are the tables:

Customers Cust_ID FirstName LastName Address Phone
01 Mickey Mouse 123 Gouda St. 456-7890
02 Donald Duck 325 Eider Ln. 786-2365

Products Part_No Name Manufacturer In_Stock Price
20-45 Hammer Stanley 57 3.50
21-68 ScrewDriver DeVries 84 2.75

Orders Order_No Part_No Cust_ID Date Quantity
2005-27 21-68 02 31 Oct 2005 2
2005-34 20-45 01 02 Nov 2005 3

We can obtain information on who has ordered what:

SELECT Customers.FirstName, Customers.LastName, Products.Name
FROM Customers, Products, Orders
WHERE Customers.Cust_ID = Orders.Cust_ID AND Products.Part_No = Orders.Part_No

We can select data from two tables with INNER JOIN. The INNER JOIN returns all rows from both tables where there is a match. If there are rows in Customers that do not have matches in Orders, those rows will not be listed.
SELECT Customers.FirstName, Customers.LastName, Orders.Date
FROM Customers
INNER JOIN Orders
ON Customers.Cust_ID = Orders.Cust_ID

The LEFT JOIN returns all the rows from the first table (Customers), even if there are no matches in the second table (Orders). If there are rows in Customers that do not have matches in Orders, those rows also will be listed.
SELECT Customers.FirstName, Customers.LastName, Orders.Date
FROM Customers
LEFT JOIN Orders
ON Customers.Cust_ID = Orders.Cust_ID

The RIGHT JOIN returns all the rows from the second table (Orders), even if there are no matches in the first table (Customers). If there had been any rows in Orders that did not have matches Customers, those rows also would have been listed.
SELECT Customers.FirstName, Customers.LastName, Orders.Date
FROM Customers
RIGHT JOIN Orders
ON Customers.Cust_ID = Orders.Cust_ID
ALTER TABLE

With ALTER TABLE you can add or delete columns in an existing table. When you add a column you must specify a data type.
ALTER TABLE table_name
ADD col_name datatype

ALTER TABLE table_name
DROP COLUMN col_name
UNION

The UNION command is used to select data from two tables very similar to the JOIN command. But the UNION command can be used only with columns having the same datatype. With UNION only distinct values are selected, i.e. if there are common data in the two tables only one instance of that data is returned.

SELECT Name FROM Customers_USA
UNION
SELECT Name FROM Customers_Asia

This will select all the customers from USA and Asia but if there is a name that occurs in both the tables it will return only one such name. To get all the names use UNION ALL instead.

SQL Functions

There are several built-in functins in SQL. The basic function types are:

Aggregate Functions: These are functions that operate against a collection of values, but return a single value.
Scalar Functions: These functions operate against a single value, and return a single value.

To use a built-in function the syntax is:

SELECT function (col_name) FROM table_name

GROUP BY

The GROUP BY was added to SQL so that aggregate functions could return a result grouped by column values.

SELECT col_name, function (col_name) FROM table_name GROUP BY col_name

HAVING keyword was introduced because the WHERE keyword could not be used. HAVING states a condition.

SELECT clo_name, function (col_name) FROM table_name
GROUP BY col_name
HAVING function (col_name) condition value
CREATE VIEW

A view is a virtual table that is a result of SQL SELECT statement. A view contains fields from one or more real tables in the database. This virtual table can then be queried as if it were a real table.

CREATE VIEW view_name AS
SELECT col_name(s)
FROM table_name
WHERE condition

A view could be used from inside a query, a stored procedure, or from inside another view. You can add functions and joins to a view and present the data you want to the user.