Microsoft SQL Server - Lesson 05: Introduction to Data Selecting

	Introduction to Data Selecting

Fundamentals of Data Selection

Introduction

Consider a table as follows:

CREATE TABLE Students
(
    StudentNumber VARCHAR(10),
    LastName VARCHAR(20)
);
INSERT Students VALUES('279227', 'Henry'),
                      ('502494', 'Robert');

One of the most regular operations performed on a database is to look for data or to isolate records that respond to a particular criterion. Looking for data that is conform to a criterion is referred to as querying. The result of retrieving data based on a criterion is called a query.

In this and other lessons, we will use a database named LambdaSquare1. The name itself means nothing.

The database represents a fictitious apartment complex. The building has three levels: First, second, and third.

Lambda Square

The apartments are referred to as units. These are what customers, named tenants, come to rent.

Each unit is represented with:

A unit number: Normally, this is the number on the door of the apartment. One of the characteristics of this number is the first digit that specifies the floor or level. For example, if a unit has number 104, this means the apartment is on the first level
Bedrooms: This is an integer from 0 to 3. The number 0 means the unit is an efficiency: A one-room apartment (the kitchen and the living room share a unique room; there is a separate bathroom)
Bathroom: This is a nnumber that indicates the number of bathrooms in the unit
Price: This is how much a tenant would pay every month
Deposit: When a customer starts renting a room, he is usually asked to put some money down, to be optionnally reimbursed when the customer leaves
Available: This is a Boolean value that indicates whether the unit can be rented

Practical Learning: Introducing Data Selection

Start the computer and log in
Launch Microsoft SQL Server and click Connect
On the main menu, click File -> New -> Query With Current Connection

In the Query Editor, type the following code:

-- ===============================================================
-- Database:	 LambdaSquare1
-- Author:	 FunctionX
-- Date Created: Sunday, January 16, 2011
-- Description:	 This database represents an apartment building.
--		 This part of the dat2base is used to present the
--		 apartments to a customer and select the right one
-- ===============================================================
USE master
GO

CREATE DATABASE LambdaSquare1;
GO

USE LambdaSquare1;
GO
CREATE SCHEMA Presentation;
GO
CREATE TABLE Presentation.Units
(
	UnitNumber nchar(10),
	Bedrooms int,
	Bathrooms real,
	Price money,
	Deposit money,
	Available bit
);

INSERT Presentation.Units
VALUES('101', 0, 1.00, 845.00,  200.00, 0),
      ('102', 1, 1.00, 895.00,  250.00, 0),
      ('103', 1, 1.00, 925.00,  275.00, 1),
      ('104', 2, 1.00, 1050.00, 300.00, 0),
      ('105', 1, 1.00, 885.00,  250.00, 1),
      ('106', 3, 2.00, 1350.00, 425.00, 1),
      ('107', 2, 2.00, 1185.00, 400.00, 0),
      ('108', 0, 1.00, 865.00,  225.00, 1),
      ('109', 2, 1.00, 1050.00, 350.00, 1),
      ('110', 1, 1.00, 895.00,  250.00, 0),
      ('111', 1, 1.00, 895.00,  250.00, 0),
      ('112', 0, 1.00, 805.00,  200.00, 1),
      ('201', 0, 1.00, 825.00,  200.00, 1),
      ('202', 1, 1.00, 950.00,  325.00, 0),
      ('203', 1, 1.00, 885.00,  250.00, 1),
      ('204', 2, 2.00, 1125.00, 425.00, 1),
      ('205', 1, 1.00, 1055.00, 350.00, 0),
      ('206', 2, 1.00, 1165.00, 400.00, 1),
      ('207', 1, 1.00, 895.00,  250.00, 0),
      ('208', 0, 1.00, 815.00,  200.00, 1),
      ('210', 1, 1.00, 895.00,  350.00, 1),
      ('211', 2, 2.00, 925.00,  400.00, 1),
      ('212', 3, 2.00, 1280.00, 500.00, 0),
      ('301', 0, 1.00, 845.00,  200.00, 0),
      ('302', 1, 1.00, 925.00,  250.00, 0),
      ('303', 2, 1.00, 985.00,  275.00, 1),
      ('304', 2, 2.00, 1250.00, 300.00, 0),
      ('305', 1, 1.00, 945.00,  250.00, 1),
      ('306', 3, 2.00, 1350.00, 425.00, 1),
      ('307', 2, 2.00, 1285.00, 400.00, 0),
      ('308', 0, 1.00, 875.00,  225.00, 1),
      ('309', 2, 1.00, 1150.00, 350.00, 1),
      ('310', 1, 1.00, 955.00,  250.00, 0),
      ('311', 3, 2.00, 1325.00, 500.00, 0),
      ('312', 0, 1.00, 825.00,  200.00, 1);
GO

To execute, on the main menu, click Query -> Execute
In the Object Explorer, right-click the Databases node and click Refresh
Expand the Databases node
Expand LambdaSquare1 and expand its Tables node

Practical Learning: Introducing the Diagram pane

On the main menu, click Query Designer, position the mouse on Pane, and click Diagram
Click the check boxes of all fields to remove the check marks

Diagram

Practical Learning: Introducing the Criteria Pane

On the Query Designer toolbar, click the Show Criteria Pane button

Column Selection in the Criteria Section

Practical Learning: Introducing the SQL Section

Right-click an empty area in the Diagram pane, position the mouse on Pane, click SQL
In the SQL section, delete TOP (200)

SQL

Practical Learning: Starting PowerShell

In the Object Explorer, right-click the name of the server and click Start PowerShell (if you receive an error, skip this step)
Type SQLCMD (or sqlcmd) and press Enter
Type USE LambdaSquare1; and press Enter
Type GO and press Enter

Launching PowerShell

The Formula to SELECT

The most fundamental keyword used by SQL is SELECT. It is used to select one or more columns from a table in order to get one or more records. You must specify what to select. The primary formula to follow is:

SELECT column(s) FROM table-name;

As stated already, SQL is not case-sensitive. This means that SELECT, Select, and select represent the same word.

The SELECT and the FROM keywords are required. The column(s) represents one or more columns from a table. The table-name represents the name of the table that has the column(s).

SELECTing ALL Columns from a Table

To select everything from a table, you can use the asterisk in the column(s) placeholder of our formula. For example, to select all records from a table named Students, you can type:

SELECT * FROM Students;

Alternatively, you can precede the asterisk with the ALL keyword.

Practical Learning: Selecting All Fields

In the PowerShell window, type:
```
SELECT ALL * FROM Presentation.Units;
```
Press Enter and type:
```
GO
```

Press Enter

PS SQLSERVER:\SQL\SIRIOUS\DEFAULT> SQLCMD
1> USE LambdaSquare1;
2> GO
Changed database context to 'LambdaSquare1'.
1> SELECT ALL * FROM Presentation.Units;
2> GO
UnitNumber   Bedrooms    Bathrooms        Price         Deposit      Available
---------- ----------- -------------- ----------------- ------------ ---------
101             0          1.0          845.0000        200.0000         0
102             1          1.0          895.0000        250.0000         0
103             1          1.0          925.0000        275.0000         1
104             2          1.0         1050.0000        300.0000         0
105             1          1.0          885.0000        250.0000         1
106             3          2.0         1350.0000        425.0000         1
107             2          2.0         1185.0000        400.0000         0
108             0          1.0          865.0000        225.0000         1
109             2          1.0         1050.0000        350.0000         1
110             1          1.0          895.0000        250.0000         0
111             1          1.0          895.0000        250.0000         0
112             0          1.0          805.0000        200.0000         1
201             0          1.0          825.0000        200.0000         1
202             1          1.0          950.0000        325.0000         0
203             1          1.0          885.0000        250.0000         1
204             2          2.0         1125.0000        425.0000         1
205             1          1.0         1055.0000        350.0000         0
206             2          1.0         1165.0000        400.0000         1
207             1          1.0          895.0000        250.0000         0
208             0          1.0          815.0000        200.0000         1
210             1          1.0          895.0000        350.0000         1
211             2          2.0          925.0000        400.0000         1
212             3          2.0         1280.0000        500.0000         0
301             0          1.0          845.0000        200.0000         0
302             1          1.0          925.0000        250.0000         0
303             2          1.0          985.0000        275.0000         1
304             2          2.0         1250.0000        300.0000         0
305             1          1.0          945.0000        250.0000         1
306             3          2.0         1350.0000        425.0000         1
307             2          2.0         1285.0000        400.0000         0
308             0          1.0          875.0000        225.0000         1
309             2          1.0         1150.0000        350.0000         1
310             1          1.0          955.0000        250.0000         0
311             3          2.0         1325.0000        500.0000         0
312             0          1.0          825.0000        200.0000         1

(35 rows affected)
1>

(If PowerShell is not working, in SQL Server Management Studio, click the SQLQuery1.sql tab

Press Ctrl + A to select everything

Type the following:

SELECT ALL * FROM Presentation.Units;
GO

To execute, press F5)

Selecting Records in PHP

Introduction

To get records using PHP, create a SELECT statement and pass it either the mysqli_query() function or the mysqli::query() method. Here is an example:

$link->query("SELECT * FROM StaffMembers")

The Number of Records Selected

To let you get the number of records of a SELECT statement, PHP provides a function named mysqli_num_rows.

Fetching the Results of a Query

To let you explore the records produced by a SELECT statement, PHP provides a function named mysqli_fetch_array. Its syntax is:

mixed mysqli_fetch_array(mysqli_result $result [, int $resulttype = MYSQLI_BOTH ])

To support the records of a table, PHP is equipped with a class named mysqli_result. The alternative to the mysqli_fetch_array function is a method named fetch_array from the mysqli_result class. Its syntax is:

mixed mysqli_result::fetch_array ([int $resulttype = MYSQLI_BOTH ] )

Both the the mysqli_fetch_array function and the mysqli_result::fetch_array method produce a collection, as an array, of records from a SELECT statement. As a result, each column can be accessed using the square brackets applied to the returned value. The mysqli_fetch_array() function or the mysqli_result::fetch_array() method takes a constant value as argument. The argument can be:

MYSQLI_NUM: Each column will be accessed by its ordinal number. The first column can be accessed using 0. The second column can be accessed with 1, and so on
MYSQLI_ASSOC: Each column will be accessed by its name provided as a string
MYSQLI_BOTH: A column can be accessed either by its ordinal number or its name

Here is an example:

<!DOCTYPE html>
<html>
<head>
<title>Students Grades - Scales</title>
</head>
<body>
<h2>Students Grades - Scales</h2>

<?php
$link = mysqli_connect("localhost", "username", "P@s$w0rd1", "DBExercise");

mysqli_connect_errno();

$link->query("CREATE TABLE GradesScales
              (
                  LetterGrade CHAR,
                  Descriptor VARCHAR(25)
              );");

$link->query("INSERT INTO GradesScales(LetterGrade, Descriptor)
              VALUES('A', 'Excellent'),
                    ('B', 'Good'),
                    ('C', 'Satisfactory'),
                    ('D', 'Acceptable'),
                    ('F', 'Failed'),
                    ('W', 'Withdrawn');");

$grades = $link->query("SELECT * FROM GradesScales");

$number_of_records = mysqli_num_rows($grades);

echo "<table border=2><tr>";
echo "<td><b>Letter Grade</b></td>";
echo "<td><b>Descriptor</b></td></tr>";

while ($grade = $grades->fetch_array(MYSQLI_ASSOC))
{
    echo "<tr><td>" . $grade["LetterGrade"] . "</td>";
    echo "<td>" . $grade["Descriptor"] . "</td></tr>";
}

echo "</table>";

$grades->close();
$link->close();
?>

</body>
</html>

This would produce:

Columns Selection

Qualifying the Names of Columns

Qualifying the name(s) of (a) column(s) consists of indicating what table to which it (they) belongs. The primary way to qualify the name of a column is to precede it with the name of the table followed by a period.

If you are using the asterisk * to select all records, precede it with the name of the table followed by a period. You must not qualify the columns in the square brackets of the array that accesses each column. Here is an example:

<!DOCTYPE html>
<html>
<head>
<title>Students Grades - Scales</title>
</head>
<body>
<h2>Students Grades - Scales</h2>

<?php
$link = mysqli_connect("localhost", "username", "P@s$w0rd1", "DBExercise");

mysqli_connect_errno();

$grades = $link->query("SELECT GradesScales.*
                        FROM GradesScales");

echo "<table border=3><tr>";
echo "	  <td>&nbsp;</td><td colspan='2' align='center'><b>Range</b></td>";
echo "	  <td colspan='2' align='center'><b>Grade Percentage</b></td>";
echo "	  <td>&nbsp;</td>";
echo "	</tr><tr><td><b>Letter Grade</b></td>";
echo "    <td><b>Minimum</b></td><td><b>Maximum</b></td>";
echo "    <td><b>Minimum</b></td><td><b>Maximum</b></td>";
echo "	  <td><b>Descriptor</b></td></tr>";

while($grade = $grades->fetch_array(MYSQLI_ASSOC))
{
    echo "<tr><td>" . $grade["LetterGrade"] . "</td>";
    echo "<td>" . $grade["MinRange"] . "</td>";
    echo "<td>" . $grade["MaxRange"] . "</td>";
    echo "<td>" . $grade["MinPercent"] . "</td>";
    echo "<td>" . $grade["MaxPercent"] . "</td>";
    echo "<td>" . $grade["Descriptor"] . "</td></tr>";
}

echo "</table>";

$grades->close();
$link->close();
?>

</body>
</html>

Practical Learning: Qualifying the Names of Columns

In the PowerShell window, type
```
SELECT ALL Presentation.Units.*
```
Press Enter and type
```
FROM Presentation.Units;
```
Press Enter and type:
```
GO
```
Press Enter
(If PowerShell is not working, in the SQL Server Management Studio, change the statement as follows:
```
SELECT ALL Presentation.Units.* 
FROM Presentation.Units;
GO
```
To execute, press F5)

The Alias Name of a Table

An alias is another name for an object. You can create an alias name for a table to use in an expression that involves a column. To create an alias of a table, first type the name of the table, followed by space, and followed by a letter or a word. An example would be GradesScales gs. Here is an example:

SELECT * FROM GradesScales gs;

You can qualify the column using the alias. This can be done as follows:

<!DOCTYPE html>
<html>
<head>
<title>Students Grades - Scales</title>
</head>
<body>
<h2>Students Grades - Scales</h2>

<?php
$link = mysqli_connect("localhost", "username", "P@s$w0rd1", "DBExercise");

mysqli_connect_errno();

$grades = $link->query("SELECT gs.*
                        FROM GradesScales gs");

echo "<table border=3><tr>";
echo "	  <td>&nbsp;</td><td colspan='2' align='center'><b>Range</b></td>";
echo "	  <td colspan='2' align='center'><b>Grade Percentage</b></td>";
echo "	  <td>&nbsp;</td>";
echo "	</tr><tr><td><b>Letter Grade</b></td>";
echo "    <td><b>Minimum</b></td><td><b>Maximum</b></td>";
echo "    <td><b>Minimum</b></td><td><b>Maximum</b></td>";
echo "	  <td><b>Descriptor</b></td></tr>";

while($grade = $grades->fetch_array(MYSQLI_ASSOC))
{
    echo "<tr><td>" . $grade["LetterGrade"] . "</td>";
    echo "<td>" . $grade["MinRange"] . "</td>";
    echo "<td>" . $grade["MaxRange"] . "</td>";
    echo "<td>" . $grade["MinPercent"] . "</td>";
    echo "<td>" . $grade["MaxPercent"] . "</td>";
    echo "<td>" . $grade["Descriptor"] . "</td></tr>";
}

echo "</table>";

$grades->close();
$link->close();
?>

</body>
</html>

Practical Learning: Using the Alias Name of a Table

In the PowerShell window, type:
```
SELECT ALL aparts.*
```
Press Enter and type:
```
FROM Presentation.Units aparts;
```
Press Enter and type:
```
GO
```
Press Enter
(If PowerShell is not working, in the SQL Server Management Studio, change the statement as follows:
```
SELECT ALL aparts.* 
FROM Presentation.Units aparts;
GO
```
To execute, press F5)
In the SQL Server Management Studio, click the Presentation.Units tab
Click the check box of * (All Columns). If a message box comes up, read it

Then click OK and click the * check box again
On the main menu, click Query Designer -> Execute SQL to see the result:
In the Diagram pane, click the check box of * (All Columns) to remove it

Practical Learning: Using a Synonym

In the PowerShell window, type:
```
CREATE SYNONYM Apartments
```
Press Enter and type:
```
FOR Presentation.Units;
```
Press Enter
Type GO and press Enter
To use the synonym, type
```
SELECT * FROM Apartments;
```
Press Enter
Type GO and press Enter
To use an alias of a synonym, type
```
SELECT apts.* FROM Apartments apts;
```
Type GO and press Enter
Return to SQL Server Managerment Studio

Techniques of Making SELECTions

SELECTing One Column

As opposed to selecting all columns from a table, you may be interested in only one particular column. To select a column, you can replace the column(s) placeholder of our formula with the name of the desired column. Here is an example:

<!DOCTYPE html>
<html>
<head>
<title>Students Grades - Scales</title>
</head>
<body>
<h2>Students Grades - Scales</h2>

<?php
$link = mysqli_connect("localhost", "username", "P@s$w0rd1", "DBExercise");

mysqli_connect_errno();

$students = $link->query("SELECT StudentNumber
                        FROM StaffMembers;");

while($staff = $students->fetch_array(MYSQLI_ASSOC))
{
    echo "Student #: " . $staff["StudentNumber"] . "<br>";
}

$students->close();
$link->close();
?>

</body>
</html>

You can also qualify a column by preceding it with the name of the table followed by the period operator. Here is an example:

SELECT StaffMembers.StudentNumber
                        FROM StaffMembers;

Practical Learning: Selecting One Field

In the Diagram pane, click the check box of Price
On the Query Designer toolbar, click the Execute SQL button to see the result:

Notice that it produces 35 records and some prices appear more than once, which indicate that some appartments have the same price

In the PowerShell window, type:

SELECT aparts.UnitNumber FROM Presentation.Units aparts;

Press Enter
Type GO and press Enter
To use the synonym, type
```
SELECT Bedrooms FROM Apartments;
```
Press Enter
Type GO and press Enter

To use an alias of a synonym, type

SELECT apts.Bathrooms FROM Apartments apts;

Type GO and press Enter

Distinct Column Selection

If you specify a column to select from a table, every record would come up. This can cause the same value to repeat over and over. Here is an example:

<!DOCTYPE html>
<html>
<head>
<title>Students Grades - Scales</title>
</head>
<body>
<h2>Students Grades - Scales</h2>

<?php
$link = mysqli_connect("localhost", "username", "P@s$w0rd1", "DBExercise");

mysqli_connect_errno();

$grades = $link->query("SELECT Descriptor
                        FROM GradesScales28");

while($grade = $grades->fetch_array(MYSQLI_ASSOC))
{
    echo "Grade Descriptor: " . $grade["Descriptor"] . "<br>";
}

$grades->close();
$link->close();
?>

</body>
</html>

This would produce:

Distinct Field Selection

Notice that PG-13 and R are repeated. Sometimes you want to show each value only once. To get a list of distinct values, put the DISTINCT keyword between SELECT and the name of the column. Here is an example:

<!DOCTYPE html>
<html>
<head>
<title>Students Grades - Scales</title>
</head>
<body>
<h2>Students Grades - Scales</h2>

<?php
$link = mysqli_connect("localhost", "username", "P@s$w0rd1", "DBExercise");

mysqli_connect_errno();

$grades = $link->query("SELECT DISTINCT Descriptor
                        FROM GradesScales28");

while($grade = $grades->fetch_array(MYSQLI_ASSOC))
{
    echo "Grade Descriptor: " . $grade["Descriptor"] . "<br>";
}

$grades->close();
$link->close();
?>

</body>
</html>

In most cases, you would get a better result if you select only one column. Still, you can use as many columns as you want.

Practical Learning: Producing Distinct Values

In the PowerShell window, type:

SELECT DISTINCT aparts.Bedrooms FROM Presentation.Units aparts;

Press Enter
Type GO and press Enter

This indicates that we have efficiencies (apartments with no real bedrooms) or apartments with 1, 2, or 3 bedrooms
To close PowerShell, type exit and press Enter
Type exit again and press Enter
In SQL Server Managerment Studio, in the SQL section, between SELECT and Price, type DISTINCT
On the Query Designer toolbar, click the Execute SQL button to see the result:
Notice that, this time, each price appears only once and, as a result, you get (only) 24 records.
In the Diagram pane, remove the check box on price. In the SQL pane, delete DISTINCT

SELECTing Many Fields of the Same Table

To consider more than one column in a statement, you can list them in the column(s) placeholder of our formula, separating them with commas. The syntax you would use is:

SELECT column1, column2, column_n FROM table-name;

Here an example:

<!DOCTYPE html>
<html>
<head>
<title>Students Grades - Scales</title>
</head>
<body>
<h2>Students Grades - Scales</h2>

<?php
$link = mysqli_connect("localhost", "username", "P@s$w0rd1", "DBExercise");

mysqli_connect_errno();

$grades = $link->query("SELECT LetterGrade,
                               MinRange,
                               MaxRange, 
                               MinPercent,
                               MaxPercent,
                               Descriptor
                        FROM GradesScales28");

echo "<table border=3><tr>";
echo "	  <td>&nbsp;</td><td colspan='2' align='center'><b>Range</b></td>";
echo "	  <td colspan='2' align='center'><b>Grade Percentage</b></td>";
echo "	  <td>&nbsp;</td>";
echo "	</tr><tr><td><b>Letter Grade</b></td>";
echo "    <td><b>Minimum</b></td><td><b>Maximum</b></td>";
echo "    <td><b>Minimum</b></td><td><b>Maximum</b></td>";
echo "	  <td><b>Descriptor</b></td></tr>";

while($grade = $grades->fetch_array(MYSQLI_ASSOC))
{
    echo "<tr><td>" . $grade["LetterGrade"] . "</td>";
    echo "<td>" . $grade["MinRange"] . "</td>";
    echo "<td>" . $grade["MaxRange"] . "</td>";
    echo "<td>" . $grade["MinPercent"] . "</td>";
    echo "<td>" . $grade["MaxPercent"] . "</td>";
    echo "<td>" . $grade["Descriptor"] . "</td></tr>";
}

echo "</table>";

$grades->close();
$link->close();
?>

</body>
</html>

You can qualify each column by preceding it with the name of the table followed by the period operator. Here is an example:

SELECT GradesScales.LetterGrade,
            GradesScales.MinRange,
            GradesScales.MaxRange, 
            GradesScales.MinPercent,
            GradesScales.MaxPercent,
            GradesScales.Descriptor
FROM GradesScales

You don't have to qualify all columns, you can qualify some and not qualify some others. You can use an alias name for a table by preceding each column with the stated alias and a period. Here is an example:

SELECT scales.LetterGrade,
            scales.MinRange,
            scales.MaxRange, 
            scales.MinPercent,
            scales.MaxPercent,
            scales.Descriptor
FROM GradesScales28 scales;

Practical Learning: Selecting Data

To get a list of the unit numbers and their prices, in the Diagram pane, click the check boxes of UnitNumber and Price (if you receive an error message box, read it and click OK, then click those check boxes again)
Right-click somewhere in the window and click Execute SQL to execute
In the Diagram pane, remove all check boxes
To get a list of the units, their number of bedrooms, their prices, and their availability, click the check boxes of UnitNumber, Bedrooms, Price, and Available
Right-click somewhere in the window and click Execute SQL

SELECTing Columns From Different Tables

You can create a SELECT statement that selects columns from more than one table of a database. Neither the tables nor the columns need to have anything in common. The formula to follow is:

SELECT column(s) FROM table_1, table_2, table_n

You start with SELECT followed by the list of columns from the tables. If the tables have columns with different names, you can simply list the name of each column. Here is an example:

<!DOCTYPE html>
<html>
<head>
<title>Students Grades - Scales</title>
</head>
<body>
<h2>Students Grades - Scales</h2>

<?php
$link = mysqli_connect("localhost", "username", "P@s$w0rd1", "DBExercise");

mysqli_connect_errno();

$grades = $link->query("SELECT LetterGrade,
                               StudentNumber,
                               MinRange,
                               MaxRange, 
                               MinPercent,
                               MaxPercent,
                               LastName,
                               Descriptor
                        FROM GradesScales28, StaffMembers;");

echo "<table border=3><tr>";
echo "	  <td>&nbsp;</td><td colspan='2' align='center'><b>Range</b></td>";
echo "	  <td colspan='2' align='center'><b>Grade Percentage</b></td>";
echo "	  <td colspan='3'>&nbsp;</td>";
echo "	</tr><tr><td><b>Letter Grade</b></td>";
echo "    <td><b>Minimum</b></td><td><b>Maximum</b></td>";
echo "    <td><b>Minimum</b></td><td><b>Maximum</b></td>";
echo "	  <td><b>Descriptor</b></td>";
echo "    <td><b>Student #</b></td>";
echo "	  <td><b>Last Name</b></td></tr>";

while($grade = $grades->fetch_array(MYSQLI_ASSOC))
{
    echo "<tr><td>" . $grade["LetterGrade"] . "</td>";
    echo "<td>" . $grade["MinRange"] . "</td>";
    echo "<td>" . $grade["MaxRange"] . "</td>";
    echo "<td>" . $grade["MinPercent"] . "</td>";
    echo "<td>" . $grade["MaxPercent"] . "</td>";
    echo "<td>" . $grade["Descriptor"] . "</td>";
    echo "<td>" . $grade["StudentNumber"] . "</td>";
    echo "<td>" . $grade["LastName"] . "</td></tr>";
}

echo "</table>";

$grades->close();
$link->close();
?>

</body>
</html>

When you select fields from different tables, in the result, each of the records of the first table would display, each showing the first record (combination of the selected columns) of the second table. Then each of the records of the first table would show again, followed by the second record (combination of the selected columns) of the second table. This will continue until all records of the second table have displayed. Consequently, the resulting query would contain (Number of Records of First Table) x (Number of Records of Second Table). For example, if the first table contains 4 records and the second table contains 2 records, the statement would produce 4 x 2 = 8 records. The above statement would produce:

Selecting Fields From Different Tables