Basic SQL Queries

Learn the fundamental SQL commands to retrieve and filter data from databases.

The SELECT Statement

The SELECT statement is used to retrieve data from a database. It's the most commonly used SQL command.

Basic Syntax

SELECT column1, column2, ...
FROM table_name;

Example Dataset

Throughout this tutorial, we'll use a students table:

+----+-----------+-----+-------+-------+
| id | name      | age | grade | city  |
+----+-----------+-----+-------+-------+
| 1  | Alice     | 20  | A     | NYC   |
| 2  | Bob       | 22  | B     | LA    |
| 3  | Charlie   | 21  | A     | NYC   |
| 4  | Diana     | 23  | C     | Chicago|
| 5  | Eva       | 20  | B     | LA    |
+----+-----------+-----+-------+-------+

Selecting Specific Columns

Select One Column

SELECT name FROM students;

Result:

+-----------+
| name      |
+-----------+
| Alice     |
| Bob       |
| Charlie   |
| Diana     |
| Eva       |
+-----------+

Select Multiple Columns

SELECT name, age, grade FROM students;

Result:

+-----------+-----+-------+
| name      | age | grade |
+-----------+-----+-------+
| Alice     | 20  | A     |
| Bob       | 22  | B     |
| Charlie   | 21  | A     |
| Diana     | 23  | C     |
| Eva       | 20  | B     |
+-----------+-----+-------+

Select All Columns

SELECT * FROM students;

The * (asterisk) selects all columns from the table.

The WHERE Clause

The WHERE clause filters records based on specified conditions.

Basic Syntax

SELECT column1, column2
FROM table_name
WHERE condition;

Examples

Filter by exact match:

SELECT * FROM students
WHERE grade = 'A';

Result:

+----+-----------+-----+-------+-------+
| id | name      | age | grade | city  |
+----+-----------+-----+-------+-------+
| 1  | Alice     | 20  | A     | NYC   |
| 3  | Charlie   | 21  | A     | NYC   |
+----+-----------+-----+-------+-------+

Filter by numeric comparison:

SELECT name, age FROM students
WHERE age > 21;

Result:

+-----------+-----+
| name      | age |
+-----------+-----+
| Bob       | 22  |
| Diana     | 23  |
+-----------+-----+

Comparison Operators

Operator	Description	Example
`=`	Equal to	`age = 20`
`!=` or `<>`	Not equal to	`grade != 'C'`
`>`	Greater than	`age > 21`
`<`	Less than	`age < 22`
`>=`	Greater than or equal	`age >= 21`
`<=`	Less than or equal	`age <= 22`

Logical Operators

AND Operator

Both conditions must be true:

SELECT * FROM students
WHERE grade = 'A' AND city = 'NYC';

Result:

+----+-----------+-----+-------+-------+
| id | name      | age | grade | city  |
+----+-----------+-----+-------+-------+
| 1  | Alice     | 20  | A     | NYC   |
| 3  | Charlie   | 21  | A     | NYC   |
+----+-----------+-----+-------+-------+

OR Operator

At least one condition must be true:

SELECT * FROM students
WHERE city = 'NYC' OR city = 'LA';

Result:

+----+-----------+-----+-------+-------+
| id | name      | age | grade | city  |
+----+-----------+-----+-------+-------+
| 1  | Alice     | 20  | A     | NYC   |
| 2  | Bob       | 22  | B     | LA    |
| 3  | Charlie   | 21  | A     | NYC   |
| 5  | Eva       | 20  | B     | LA    |
+----+-----------+-----+-------+-------+

NOT Operator

Negates a condition:

SELECT * FROM students
WHERE NOT grade = 'C';

ORDER BY Clause

Sort results in ascending or descending order.

Ascending Order (Default)

SELECT * FROM students
ORDER BY age;

Result:

+----+-----------+-----+-------+-------+
| id | name      | age | grade | city  |
+----+-----------+-----+-------+-------+
| 1  | Alice     | 20  | A     | NYC   |
| 5  | Eva       | 20  | B     | LA    |
| 3  | Charlie   | 21  | A     | NYC   |
| 2  | Bob       | 22  | B     | LA    |
| 4  | Diana     | 23  | C     | Chicago|
+----+-----------+-----+-------+-------+

Descending Order

SELECT * FROM students
ORDER BY age DESC;

Multiple Columns

SELECT * FROM students
ORDER BY grade ASC, age DESC;

LIMIT Clause

Restrict the number of rows returned:

SELECT * FROM students
LIMIT 3;

Result:

+----+-----------+-----+-------+-------+
| id | name      | age | grade | city  |
+----+-----------+-----+-------+-------+
| 1  | Alice     | 20  | A     | NYC   |
| 2  | Bob       | 22  | B     | LA    |
| 3  | Charlie   | 21  | A     | NYC   |
+----+-----------+-----+-------+-------+

DISTINCT Keyword

Remove duplicate values:

SELECT DISTINCT city FROM students;

Result:

+---------+
| city    |
+---------+
| NYC     |
| LA      |
| Chicago |
+---------+

Practice Exercises

Try these queries on your own:

Select all students from NYC
Find students aged 20 or 21
Get names of students with grade A, sorted alphabetically
Find the 2 oldest students
List all unique grades in the table

Solutions

-- 1. Students from NYC
SELECT * FROM students WHERE city = 'NYC';

-- 2. Students aged 20 or 21
SELECT * FROM students WHERE age = 20 OR age = 21;

-- 3. Grade A students, sorted by name
SELECT name FROM students WHERE grade = 'A' ORDER BY name;

-- 4. Two oldest students
SELECT * FROM students ORDER BY age DESC LIMIT 2;

-- 5. Unique grades
SELECT DISTINCT grade FROM students;

SQL is Case-Insensitive

SQL keywords are not case-sensitive. SELECT, select, and SeLeCt all work the same. However, it's a good practice to write keywords in UPPERCASE for readability.

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