SQL, ‘Structured Query Language’, is a programming language designed to manage data stored in relational databases. SQL operates through simple, declarative statements. This keeps data accurate and secure, and helps maintain the integrity of databases, regardless of size.
Here’s an appendix of commonly used commands.
ALTER TABLE table_name ADD column datatype;
ALTER TABLE lets you add columns to a table in a database.
SELECT column_name(s) FROM table_name WHERE column_1 = value_1 AND column_2 = value_2;
AND is an operator that combines two conditions. Both conditions must be true for the row to be included in the result set.
SELECT column_name AS 'Alias' FROM table_name;
AS is a keyword in SQL that allows you to rename a column or table using an alias.
SELECT AVG(column_name) FROM table_name;
AVG() is an aggregate function that returns the average value for a numeric column.
SELECT column_name(s) FROM table_name WHERE column_name BETWEEN value_1 AND value_2;
BETWEEN operator is used to filter the result set within a certain range. The values can be numbers, text or dates.
SELECT COUNT(column_name) FROM table_name;
COUNT() is a function that takes the name of a column as an argument and counts the number of rows where the column is not
CREATE TABLE table_name (column_1 datatype, column_2 datatype, column_3 datatype);
CREATE TABLE creates a new table in the database. It allows you to specify the name of the table and the name of each column in the table.
DELETE FROM table_name WHERE some_column = some_value;
DELETE statements are used to remove rows from a table.
SELECT COUNT(*) FROM table_name GROUP BY column_name;
GROUP BY is a clause in SQL that is only used with aggregate functions. It is used in collaboration with the
SELECT statement to arrange identical data into groups.
SELECT column_name(s) FROM table_1 JOIN table_2 ON table_1.column_name = table_2.column_name;
An inner join will combine rows from different tables if the join condition is true.
INSERT INTO table_name (column_1, column_2, column_3) VALUES (value_1, 'value_2', value_3);
INSERT statements are used to add a new row to a table.
SELECT column_name(s) FROM table_name WHERE column_name LIKE pattern;
LIKE is a special operator used with the
WHERE clause to search for a specific pattern in a column.
SELECT column_name(s) FROM table_name LIMIT number;
LIMIT is a clause that lets you specify the maximum number of rows the result set will have.
SELECT MAX(column_name) FROM table_name;
MAX() is a function that takes the name of a column as an argument and returns the largest value in that column.
SELECT MIN(column_name) FROM table_name;
MIN() is a function that takes the name of a column as an argument and returns the smallest value in that column.
SELECT column_name FROM table_name WHERE column_name = value_1 OR column_name = value_2;
OR is an operator that filters the result set to only include rows where either condition is true.
SELECT column_name FROM table_name ORDER BY column_name ASC|DESC;
ORDER BY is a clause that indicates you want to sort the result set by a particular column either alphabetically or numerically.
SELECT column_name(s) FROM table_1 LEFT JOIN table_2 ON table_1.column_name = table_2.column_name;
An outer join will combine rows from different tables even if the the join condition is not met. Every row in the left table is returned in the result set, and if the join condition is not met, then
NULL values are used to fill in the columns from the righttable.
SELECT ROUND(column_name, integer) FROM table_name;
ROUND() is a function that takes a column name and an integer as an argument. It rounds the values in the column to the number of decimal places specified by the integer.
SELECT column_name FROM table_name;
SELECT statements are used to fetch data from a database. Every query will begin with SELECT.
SELECT DISTINCT column_name FROM table_name;
SELECT DISTINCT specifies that the statement is going to be a query that returns unique values in the specified column(s).
SELECT SUM(column_name) FROM table_name;
SUM() is a function that takes the name of a column as an argument and returns the sum of all the values in that column.
UPDATE table_name SET some_column = some_value WHERE some_column = some_value;
UPDATE statments allow you to edit rows in a table.
SELECT column_name(s) FROM table_name WHERE column_name operator value;
WHERE is a clause that indicates you want to filter the result set to include only rows where the following condition is true.
In short; AJAX is about loading data in the background and display it on the webpage, without reloading the whole page.
Examples of applications using AJAX: Gmail, Google Maps, Youtube, and Facebook tabs.
JQuery aims to ease all these problems by providing a lightweight library that adds many advanced and cross-browser functions to the standard language. In addition, there is a very dynamic community that adds more-advanced components based on JQuery.
jQuery has a large set of library/functions.
jQuery has good documentation and help manuals are easily available.
jQuery supports AJAX functionality.
You are no longer writing the native language, so your understanding of what is actually occuring may be limited.
With Ajax in a website there is no need to refresh a Web page continuously. For users who are waiting to see if they won an auction, keeping an eye on sports scores, or closely following the latest weather forecasts, Ajax enhancements to these types of Web pages can greatly improve their experiences.
AJAX pros and cons
AJAX is rapidly becoming an integral part of several websites, several well established brands online now use AJAX to handle their web applications because it provides better interactivity to their users, this is due to the fact that implementing AJAX on a website, does not require a page to be reloaded for dynamic content on web pages. While there are numerous reasons to switch to AJAX there are quite a few matters that would make you reconsider using this combination of technologies as well. Below are some of the advantages and disadvantages of using AJAX.
This is pretty much the most striking benefit behind why several developers and webmasters are switching to AJAX for their websites. AJAX allows easier and quicker interaction between user and website as pages are not reloaded for content to be displayed.
AJAX applications on websites can be built to allow easier navigation to users in comparison to using the traditional back and forward button on a browser.
With AJAX, several multi purpose applications and features can be handled using a single web page, avoiding the need for clutter with several web pages.
Backed by reputed brands
Another assuring reason to use AJAX on your websites is the fact that several complex web applications are handled using AJAX, Google Maps is the most impressive and obvious example, other powerful, popular scripts such as the vBulletin forum software has also incorporated AJAX into their latest version.
The back and refresh button are rendered useless
With AJAX, as all functions are loaded on a dynamic page without the page being reloaded or more importantly a URL being changed (except for a hash symbol maybe), clicking the back or refresh button would take you to an entirely different web page or to the beginning of what your dynamic web page was processing. This is the main drawback behind AJAX but fortunately with good programming skills this issue can be fixed.