Intro

SQL, which stands forStructured Query Language, is a language for interacting with data stored in something called arelational database.

Each row, or record, of a table contains information about a single entity. For example, in a table representing employees, each row represents a single person. Each column, or field, of a table contains a single attribute for all rows in the table. For example, in a table representing employees, we might have a column containing first and last names for all employees.

In SQL, you can select data from a table using a SELECT statement. For example, the following query selects the name column from the people table

In the real world, you will often want to select multiple columns. Luckily, SQL makes this really easy. To select multiple columns from a table, simply separate the column names with commas!

For example, this query selects two columns,name and birthdate, from thepeopletable:

Sometimes, you may want to select all columns from a table. Typing out every column name would be a pain, so there's a handy shortcut:

SELECT *

FROM people;

If you only want to return a certain number of results, you can use the LIMIT keyword to limit the number of rows returned:

SELECT *

FROM people

LIMIT 10;

Often your results will include many duplicate values. If you want to select all the unique values from a column, you can use the DISTINCT keyword.

This might be useful if, for example, you're interested in knowing which languages are represented in thefilmstable:

SELECT DISTINCT language

FROM films;

What if you want to count the number of employees in your employees table? The COUNT statement lets you do this by returning the number of rows in one or more columns.

For example, this code gives the number of rows in thepeopletable:

SELECT COUNT(*)

FROM people;

As you've seen, COUNT(*) tells you how many rows are in a table. However, if you want to count the number of non-missing values in a particular column, you can call COUNT on just that column.

It's also common to combine COUNT with DISTINCT to count the number of distinct values in a column.