SQL Basics & WHERE Clause Introduction

SQL (Structured Query Language) is a standard language designed for managing data held within a relational database management system (RDBMS), or for stream processing in a relational data stream management system (RDSMS). This tutorial gives an introduction to SQL Basics with a focus on the WHERE clause.

1. Setting Up The Environment

Before you start practicing SQL commands, you need to setup an environment. You can download and install MySQL Server and MySQL Workbench. This guide will be based on MySQL.

Step 1 - Install MySQL Server

You can download the MySQL installer using the following link MySQL Installer. After downloading, follow the instructions in this link MySQL Server Installation Guide to install.

Step 2 - Install MySQL Workbench

You can download MySQL Workbench using the following link MySQL Workbench. Follow the instructions to install it.

2. SQL Basics

- CREATE Statement

The CREATE statement is used to create a new table in a database.

CREATE TABLE Employees (
    ID int,
    Name varchar(255),
    Age int,
    City varchar(255));

- INSERT Statement

The INSERT INTO statement is used to insert new records in a table.

INSERT INTO Employees (ID, Name, Age, City)
VALUES ('1', 'John', '27', 'New York');

That will insert a new row into the Employees table.

3. WHERE Clause Introduction

The WHERE clause is used to filter records. It extracts only those records that fulfill a specified condition.

SQL WHERE Syntax

SELECT column1, column2, ...
FROM table_name 
WHERE condition;

Example

Assume we've the following data in our Employees table:

If you want to select all the employees who are from New York, the SQL query would be:

SELECT * FROM Employees
WHERE City='New York';

This will return:

You can also use multiple conditions in your WHERE clause:

SELECT * FROM Employees
WHERE City='New York' AND Age='27';

This will return:

That covers the SQL Basics and especially the WHERE clause. There are many more SQL commands and the WHERE clause can be much more complex but this should serve as a good starting point.

Working with Single Condition Filtering

The 'WHERE' clause allows you to filter results from your SQL statements. It can be used in various kinds of SQL statements such as SELECT, UPDATE, DELETE, etc. In this guide, we'll focus on using it within the SELECT statement. This section is a hands on guide that provides practical implementations on filtering SQL queries using the WHERE clause with a single condition.

Sample Data

In order to begin, we'll assume the availability of a dataset. We will use a simplified version of the 'employees' table for ease of demonstration.

The 'employees' table:

Filtering Results

The structure for a basic SELECT statement which uses the WHERE clause can be seen below:

SELECT column1, column2, ..
FROM table_name
WHERE column N condition;

Let's put this into practice using real examples from our employees database.

1. Filtering Based on Text Conditions:

You may want to filter all employees who live in the USA:

SELECT * 
FROM employees 
WHERE Country='USA';

This will return:

2. Filtering Based on Numeric Conditions:

You may want to find all employees with a salary greater than $65,000:

SELECT * 
FROM employees 
WHERE Salary>65000; 

This will return:

3. Filtering Based on Date Conditions:

Assuming there's a 'HireDate' column in your table (in 'yyyy-mm-dd' format), you can filter all employees hired before 2019:

SELECT * 
FROM employees 
WHERE HireDate<'2019-01-01';

Please replace examples with real DATE values according to your actual data set.

Conclusion

The WHERE clause is highly flexible and can be used to filter rows based on a single condition, providing a fine degree of control over which records are selected from the database. As we've seen, the data type of the column determines whether we use text, numeric or date values. The practical examples above illustrate how to utilize the WHERE clause with single condition filtering.

Mastering Multiple Conditions Filtering

In this section, we will be discussing and implementing multiple conditions filtering using the SQL WHERE clause. This is quite straight-forward considering we've already covered single condition filtering in part 2 of this series.

Basics of Multiple Conditions Filtering

When you want to filter data in SQL based on multiple conditions, you may use the logical operators AND and OR to chain multiple conditions together.

SELECT column1, column2, ...
FROM table_name
WHERE condition1 AND/OR condition2 AND/OR condition3 ...;

Practical Examples

Let's look at a practical scenario. Consider we have a students table with the following structure:

Table: students

Example 1: Using AND

Here's a SQL query that will select students who are from 'USA' AND have a Score greater than 80.

SELECT *
FROM students
WHERE Country = 'USA' AND Score > 80;

This should produce the following result:

Example 2: Using OR

Here's a SQL query that will select students who are either from 'Canada' OR have a Score less than 80.

SELECT *
FROM students
WHERE Country = 'Canada' OR Score < 80;

This should produce the following result:

Example 3: Using AND and OR Together

Whenever we use both AND and OR together, it's best to use parentheses () to avoid confusion and ensure the correct logical order.

In this SQL query, we will select students who are from 'Canada' AND have a score greater than 80 OR students who are from 'USA'.

SELECT *
FROM students
WHERE (Country = 'Canada' AND Score > 80) OR Country = 'USA';

This should produce the following result:

This concludes our coverage of mastering multiple conditions filtering in SQL. Happy querying!

Advanced WHERE Applications & SQL Operators

In this part, we'll focus on more advanced WHERE applications & SQL operators. We'll demonstrate the practical uses of BETWEEN, LIKE, IN and IS NULL operators with real-life data.

Let's imagine we have a database named "companydb" with a table named "employees" featuring columns "id", "name", "dob", "salary" and "department".

Utilizing BETWEEN

The BETWEEN operator in SQL is used to fetch records where column values fall within a specified range. Let's use it to select employees born between 1960 and 1970:

SELECT *
FROM employees
WHERE YEAR(dob) BETWEEN 1960 AND 1970;

This SQL statement would return all records from the employees table where the year of "dob" is between 1960 and 1970 (inclusive).

Utilizing LIKE

The LIKE operator is used for pattern matching. Let's use it to find employees whose names start with "J":

SELECT *
FROM employees
WHERE name LIKE 'J%';

The "%" character is a wildcard that represents zero, one or multiple characters. So this SQL would return all records from the employees table where the name starts with "J".

Utilizing IN

The IN operator allows you to specify multiple values in a WHERE clause providing the same functionality as multiple OR conditions. Let’s use it to specify a list of departments to fetch employees from:

SELECT *
FROM employees
WHERE department IN ('Sales', 'Marketing', 'HR');

This SQL would return all records from the employees table where the department is either 'Sales', 'Marketing', or 'HR'.

Utilizing IS NULL

The IS NULL operator is used to test for empty (null) values. Let's use it to find employees whose department is not set:

SELECT *
FROM employees
WHERE department IS NULL;

This SQL statement would return all records from the employees table where the department value is NULL.

Please replace the table name and column names with your own table name and corresponding column names from your project. Trial different conditions to understand the working of these operators better.