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 (val1val2, …)

 
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_IDFirstNameLastNameAddressPhone
01MickeyMouse123 Gouda St.4567890
02DonaldDuck325 Eider Ln.7862365

Products

Part_NoNameManufacturerIn_StockPrice
20-45HammerStanley573.50
21-68ScrewDriverDeVries842.75

Orders

Order_NoPart_NoCust_IDDateQuantity
2005-2721-680231 Oct 20052
2005-3420-450102 Nov 20053

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.

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