Working with Database

Create a database with a specified name if it does not exist in database server

CREATE DATABASE [IF NOT EXISTS] database_name

Use database or change current database to another database you are working with

USE database_name

Drop a database with specified name permanently. All physical file associated with the database is no longer exists.

DROP DATABASE [IF EXISTS] database_name

Show all available databases in database server

SHOW DATABASES

Working with Table

Lists all tables in a database.

SHOW TABLES

Create table statement defines the structure of table in a database.

 CREATE [TEMPORARY] TABLE [IF NOT EXISTS] table_name
 (create_clause ,...) [table_options]
 [[IGNORE|REPLACE] select]

Altering table structure

ALTER [IGNORE] TABLE table_name actions

Actions can be one of the following actions:

ADD [COLUMN]

Add a new column into a table

DROP [COLUMN]

Drop an existing column in a table

ADD INDEX [name](column_name, ...)

Add index with a specific name to a table on a column

DROP INDEX index_name Drop an index from a table

ADD PRIMARY KEY (column_name,…)

Add primary key into a tables

DROP PRIMARY KEY

Drop primary key from a table

Deleting table permanently

 DROP TABLE [IF EXISTS] table_name [, name2, ...]
 [RESTRICT | CASCADE]

Give information about the table or column.

DESCRIBE table [column_name]
DESC table [column_name]
 

Working with Index

Creating an index with the specified name on a table

CREATE [UNIQUE|FULLTEXT] INDEX index_name
ON table (column_name,...)

Removing a specified index from table

DROP INDEX index_name

Retrieving Data

Retrieving complete data in a database table

SELECT * FROM table_name

Retrieving specified data which is shown in the column list from a database table

SELECT column_name, column_name2….
FROM table_name

Retrieving unique records

SELECT DISTINCT (column_name)
FROM table_name

Retrieving data from multiples table using join

SELECT *
FROM table_name1
INNER JOIN table_name2 ON conditions

SELECT *
FROM table_name1
LEFT JOIN table_name2 ON conditions

SELECT *
FROM table_name1
RIGHT JOIN table_name2 ON conditions

Counting number of rows in a database table

SELECT COUNT (*)
FROM table_name

Sorting ascending or descending retrieved data based on one or more column

SELECT column_name, column_name2….
FROM table_name
ORDER BY column_name ASC [DESC], column_name2 ASC [DESC],

Group the retrieved rows data

SELECT *
FROM table_name
GROUP BY column_name

Matching Data based on a pattern

Matching data using LIKE operator

SELECT * FROM table_name
WHERE column_name LIKE ‘%value%’

Matching data using regular expression

SELECT * FROM table_name
WHERE column_name RLIKE ‘regular_expression’



 

 

Mark
 BSc Honours in Computer Science
 Software Architect / Developer
 Mobile 079 9111 855

Email info@mydeveloper.co.za

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In some cases, the information we need is not actually stored in the database but we can retrieve it by computing in some ways from stored data. For example, we have OrderDetails table to store order data, we don’t know total of money of all selling products when we look at the table. In order to do so, we can use aggregate functions. By definition, aggregate functions allow us to perform a calculation on a set of records and return a single returned value. Aggregate functions ignore null value when performing calculation except COUNT function. Aggregate functions are often used with GROUP BY clause of SELECT statement. Here are aggregate functions which MySQL supports:

SUM Function

SUM function returns the sum of all values in an expression.

Let’s practice with OrderDetails table by following examples:

To get the total money for each selling product we just use the SUM function and group by product. Here is the query:

SELECT productCode,sum(priceEach * quantityOrdered) total
FROM orderdetails
GROUP by productCode

productCode    total
-----------  ---------
S10_1678      90157.77
S10_1949     190017.96
S10_2016     109998.82
S10_4698        170686
S10_4757     127924.32
S10_4962     123123.01
S12_1099     161531.48

To see the result more details, we can join the OrderDetails table with Product table.

SELECT P.productCode,
       P.productName,
       SUM(priceEach * quantityOrdered) total
FROM orderdetails O
INNER JOIN products  P ON O.productCode = P.productCode
GROUP by productCode
ORDER BY total

productCode  productName                                    total
-----------  -------------------------------------------  ---------
S24_1937     1939 Chevrolet Deluxe Coupe                   28052.94
S24_3969     1936 Mercedes Benz 500k Roadster              29763.39
S24_2972     1982 Lamborghini Diablo                       30972.87
S24_2840     1958 Chevy Corvette Limited Edition           31627.96
S32_2206     1982 Ducati 996 R                             33268.76
S24_2022     1938 Cadillac V-16 Presidential Limousine     38449.09
S50_1341     1930 Buick Marquette Phaeton                  41599.24
S24_1628     1966 Shelby Cobra 427 S/C                     42015.54
S72_1253     Boeing X-32A JSF                              42692.53

AVG Function

AVG is used to calculate average value of an expression. It ignores NULL values.

AVG(expression)

We can use AVG function to calculate the average price of all products buy executing the following query.

SELECT AVG(buyPrice) average_buy_price
FROM Products

Here is the result

average_buy_price
-----------------
  54.395181818182

MAX and MIN Function

MAX function returns the maximum and MIN function returns the minimum value of the set of values in expression.

MAX(expression)

MIN(expression)

As an example, we can use MIN and MAX function to retrieve the highest and lowest price product as follows:

SELECT MAX(buyPrice) highest_price,
       MIN(buyPrice) lowest_price
FROM Products

You will get the result

highest_price  lowest_price
-------------  ------------
       103.42         15.91

COUNT Function

COUNT function returns the count of the items in expression. We can use COUNT function to count how many products we have as follows:

SELECT COUNT(*) AS Total
FROM products

Total
------
   110

For more information Mark on 079 9111 855 or email info@mydeveloper.co.za

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Do you know what is returned when you used the mysql_query function to query a MySQL database? It isn’t something you can directly manipulate, that is for sure. Here is a sample SELECT query of a table we created in the MySQL Create Table lesson.

<?php
$result = mysql_query("SELECT * FROM example");
?>

The value that mysql_query returns and stores into $result is a special type of data, it is a MySQL Resource. Additional PHP functions are required to extract the data from this Resource.

A Row of Data

The mysql_fetch_array function takes a MySQL query resource as an argument ($result) and returns the first row of data returned by the mysql_query. Our table example basically looks like the table below.

example MySQL Table:

The first row of data in this table is “Timmy Mellowman” and “23″. When we fetch an array from our MySQL Resource $result it should have Timmy’s name and age in it.

Getting a Row of Data using mysql_fetch_array

mysql_fetch_array returns the first row in a MySQL Resource in the form of an associative array. The columns of the MySQL Result can be accessed by using the column names of the table. In our table example these are: name and age. Here is the code to print out the first MySQL Result row.

<?php
// Make a MySQL Connection
$query = "SELECT * FROM example"; 

$result = mysql_query($query) or die(mysql_error());

$row = mysql_fetch_array($result) or die(mysql_error());
echo $row['name']. " - ". $row['age'];
?>

Display:

This is just what we expected would happen! Now, the cool thing about mysql_fetch_array is that you can use it again on the same MySQL Resource to return the second, third, fourth and so on rows. You can keep doing this until the MySQL Resource has reached the end (which would be three times in our example).

Sounds like an awfully repetitive task. It would be nice if we could get all our results from a MySQL Resource in an easy to do script.

Fetch Array While Loop

As we have said, the mysql_fetch_array function returns an associative array, but it also returns FALSE if there are no more rows to return! Using a PHP While Loop we can use this information to our advantage.

If we place the statement “$row = mysql_fetch_array()” as our while loop’s conditional statement we will accomplish two things:

1. We will get a new row of MySQL information that we can print out each time the while loop checks its conditional statement.
2. When there are no more rows the function will return FALSE causing the while loop to stop!

Now that we know what we need to do and how to go about doing it, the code pretty much writes itself, so let’s move on to the next lesson. Just kidding! Here is the code that will print out all the rows of our MySQL Resource.

<?php
// Make a MySQL Connection
$query = "SELECT * FROM example"; 

$result = mysql_query($query) or die(mysql_error());

while($row = mysql_fetch_array($result)){
	echo $row['name']. " - ". $row['age'];
	echo "<br />";
}
?>

Display:

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