rriess1 / rriess1-selectwhere

Background

SQL sublanguage: DQL (Data Query Language)

Now that we know how to query all records from a table utilizing the SELECT keyword, it might be beneficial to filter what records are given to us from a table.

For example lets look at the "employee" table below:

employee table

id	first_name	last_name	salary
1	'Steve'	'Garcia'	67400.00
2	'Alexa'	'Smith'	42500.00
3	'Steve'	'Jones'	99890.99
4	'Brandon'	'Smith'	120000.00
5	'Adam'	'Jones'	55050.50

Let's say we wanted to query all the records from the table that have the first name "Steve".

The statement that will be utilized is as follows:

SELECT * FROM employee WHERE first_name = 'Steve';

In the above statement, the WHERE keyword allows us to filter the records based on the condition defined after the keyword. The condition that we specified is that the first_name column must have the value "Steve".

This statement would have returned the following result set:

employees table

id	first_name	last_name	salary
1	'Steve'	'Garcia'	67400.00
3	'Steve'	'Jones'	99890.99

In addition to filtering on equality like above, we can filter on inequality with the <, >, <=, >=, and != operators. We can even filter on strings that match partially, using the LIKE keyword and the '%' wildcard. Here are a few examples: This will return all records where the first name starts with 'S'

SELECT * FROM employee WHERE first_name LIKE 'S%';

This will return all the records with a salary value of less than $100,000.00

SELECT * FROM employee WHERE salary < 100000.00;

Additional reference material if needed: https://www.w3schools.com/sql/sql_where.asp

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Lab

Problem 1

employee table

id	first_name	last_name	salary
1	'Steve'	'Garcia'	67400.00
2	'Alexa'	'Smith'	42500.00
3	'Steve'	'Jones'	99890.99
4	'Brandon'	'Smith'	120000.00
5	'Adam'	'Jones'	55050.50

Problem 1: Given the above employee table, query all the records from the table that have the last_name 'Smith'. Remember to use single quotes when filtering for varchars, not double quotes.

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Problem 2

employee table

id	first_name	last_name	salary
1	'Steve'	'Garcia'	67400.00
2	'Alexa'	'Smith'	42500.00
3	'Steve'	'Jones'	99890.99
4	'Brandon'	'Smith'	120000.00
5	'Adam'	'Jones'	55050.50

Problem 2: Query the employee table for all records with a salary greater than $75,000