04 - Select Statement in MySQL

https://youtu.be/HYD8KjPB9F8?si=A8GAsx6CAOo0oHRY

TOTAL: 11 MINS

02:51

Thử chọn schema khác và chạy dòng lệnh cũ -> báo lỗi do sai db active

=> nếu thao tác trên server có nhiều db cùng lúc phải gõ tendb.table

09:02

SELECT distinct gender

FROM parks_and_recreation.employee_demographics;

=> k cho trùng cột gender

-- SELECT STATEMENT

-- the SELECT statement is used to work with columns and specify what columns you want to work see in your output. There are a few other things as well that

-- we will discuss throughout this video

-- We can also select a specefic number of column based on our requirement.

-- Now remember we can just select everything by saying:

SELECT *

FROM parks_and_recreation.employee_demographics;

-- Let's try selecting a specific column

SELECT first_name

FROM employee_demographics;

-- As you can see from the output, we only have the one column here now and don't see the others

-- Now let's add some more columns, we just need to separate the columns with columns

SELECT first_name, last_name

FROM employee_demographics;

-- Now the order doesn't normall matter when selecting your columns.

-- There are some use cases we will look at in later modules where the order of the column

-- Names in the select statement will matter, but for this you can put them in any order

SELECT last_name, first_name, gender, age

FROM employee_demographics;

-- You'll also often see SQL queries formatted like this.

SELECT last_name,

first_name,

gender,

age

FROM employee_demographics;

-- The query still runs the exact same, but it is easier to read and pick out the columns

-- being selected and what you're doing with them.

-- For example let's take a look at using a calculation in the select statement

-- You can see here we have the total_money_spent - we can perform calculations on this

SELECT first_name,

 last_name,

 total_money_spent,

 total_money_spent + 100

FROM customers;

-- See how it's pretty easy to read and to see which columns we are using.

-- Math in SQL does follow PEMDAS which stands for Parenthesis, Exponent, Multiplication,

-- Division, Addition, subtraction - it's the order of operation for math

-- For example - What will the output be?:

SELECT first_name,

last_name,

salary,

salary + 100

FROM employee_salary;

-- This is going to do 10* 100 which is 1000 and then adds the original 540

-- Now what will the output be when we do this?

SELECT first_name,

last_name,

salary,

(salary + 100) * 10

FROM employee_salary;

--  Pemdas

-- One thing I wanted to show you about the select statement in this lesson is the DISTINCT Statement - this will return only unique values in

-- The output - and you won't have any duplicates

SELECT department_id

FROM employee_salary;

SELECT DISTINCT department_id

FROM employee_salary;

-- Now a lot happens in the select statement. We have an entire module dedicated to just the

-- select statement so this is kind of just an introduction to the select statement.