# Learning SQL — The SQL Cheatsheet — Part II

I’ve been taking some refresher courses on SQL for an upcoming job through **Codecademy** (I highly recommend it if you want to learn to code — one of the best platforms I’ve ever used — you can get a month for free). As I was learning it, I thought it would be handy to create a cheat sheet with the most-used basic SQL command-lines, for a quick learning-session or refresher, for me and for you.

# COUNT

Calculate how many rows are in a table.

# SUM

Returns sum of all the values in the column.

# MAX/MIN

`MAX()`

returns the largest value in a column. `MIN()`

returns the smallest value in a column.

# AVERAGE

Calculate the average value of a column.

# ROUND

Rounds the value in the column to the specified number of decimal places. `ROUND()`

takes two arguments, a column name and an integer (the decimal places).

# GROUP BY I (one column)

Used with `SELECT`

to arrange data into groups.

# GROUP BY II (two columns)

When we need to reference more than one columns, we can use column reference numbers:

`1`

is the first column selected`2`

is the second column selected`3`

is the third column selected

# HAVING

Used to filter what groups to include or exclude. `WHERE()`

can only filter *rows,* not *groups. *The query below only includes price points that have more than 10 orders placed.

# JOIN ON

Combine tables based on common column(s) and keep rows from both tables.

# INNER JOIN

Combine tables and only keep rows that match the `ON`

condition.

# LEFT JOIN

Combine tables and keep all rows from the first table regardless of whether there’s a matching row in the second table.

# PRIMARY KEY vs FOREIGN KEY

**Primary key** is a column that uniquely identifies each row of a table.

Requirements for primary keys:

- None of the values can be
`NULL`

- Each value must be unique
- A table can not have more than one primary key column

When the** primary key **for one table appears in a different table.

# CROSS JOIN

`CROSS JOIN`

combines all rows of one table with all rows of another table.

# UNION

`UNION`

is used to stack one dataset on top of the other:

- The tables must have the same number of columns
- The columns must have the same data types in the same order

# WITH

For more complicated cases when we want to combine two tables, and one of which is the result of a calculation. we can use `WITH`

to achieve it.

*Keep reading: **The SQL Cheat Sheet — Part I*

*Originally published at **https://www.goodmarketing.club** on February 12, 2021.*