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). 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 include 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.

Requirement 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
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.