SELECT

The basic statement in SQL is SELECT. We use it to retrieve tables, which is the main action of SQL queries.

It reads much like plain English: we SELECT specific columns FROM a specific table.

SELECT [columns] FROM <table>

For example, if we have the table employees:

We can SELECT any subset of columns we need.

For example, if we wanted to get a smaller table that only has the columns id, first_name, and salary, we can use the SELECT statement to get them FROM employees.

We just need to separate the names of the columns we need with commas:

SELECT id, first_name, salary 
FROM employees

The output would look like this:

If we wanted to select all the columns from a specific table, we could just use the wildcard (*)

For example, the query

SELECT * FROM employees

would just return us the whole table employees.

Finally, we can also rename columns through the AS keyword.

For example,

SELECT id, 
first_name AS name, 
last_name AS surname 
FROM employees

would retrieve us a table with three columns, with first_name renamed as name and last_name renamed as surname:

WHERE clause

The WHERE clause is a clause that lets us filter the rows returned by our SELECT statement. To use it, we must add it after the SELECT.

For example,

SELECT * FROM employees 
WHERE salary >= 80000

would return all the columns from the employees table for which salaries are greater than or equal to 80000. This leaves out the first row because it has a salary of 75000:

We can include the usual relationships in the filtering condition after the WHERE clause:

We can also use the logical connectors NOT, AND, and OR to combine conditions, so we can write queries like this:

SELECT first_name, last_name FROM employees
WHERE salary >= 80000 AND date_hired >= 2019-01-01

This would return the field’s first_name and last_name for every employee with a salary higher than 80000 hired since 2019. Only Jane Williams satisfies the two conditions, so our table will have just one row:

What we’ve learned so far:

When starting with SQL, the execution order of different clauses may be hard to follow.

Therefore, in the introductory courses, we will finish the lessons with example queries that use clauses we learned up to that point.

Then, we will show their order of execution and the actions they perform.

For now, this is it:

Example query

SELECT first_name AS name, last_name
FROM employees
WHERE salary > 7500 AND last_name != "Williams"

Execution order

1. FROM -> Gets the input table.
2. WHERE -> Filters rows according to a condition.
3. SELECT -> Selects the columns we need.
4. AS -> Renames columns.