A place to learn some SQL!

Now, I particularly use PostgreSQL, using pgAdmin 4 dashboard. You could use Mysql, or other sorts of databases that run sql queries. It doesn't matter. However, there are slight differences in each and I suggest doing research to figure out which one is best for you and your situation.

Wanna download it? Go here. Here's a great website to learn many commands.

Upon setup, you will need to create a password. DO NOT FORGET THAT PASSWORD. You will need it to be able to use pg admin.

Let's go....

Using mockstaff.sql file

You can run this query to see information about a specific table. Once it's downloaded and opened in postgreSQL.

SELECT * FROM company_divisions

The select is a function stating you want to select something. The star means to grab all of information stored and company_divisions is a table that we have in our database. We want to extract information (all of it, hence the star) from a specific table.

SELECT * FROM company_regions

Returns us (all) the information from the table company regions!. Hence the * We did not place restrictions on what information we want requested. The star just says give us everything.

Now you could imagine you could just change company_regions with any other table.

Using the count_min_max.sql file and mockstaff.file should be already loaded.

Okay. We have a large table! What if you don't want to see eveything at once... What if you wanted to see just a hint of what the whole table shows..

SELECT * FROM staff LIMIT 10

There you go. Using the LIMIT clause, you should see 10 rows from the table returned instead of 1000 that is in the table.

Now once you load the file and run the query say, Select * FROM staff A notification will appear in pgadmin showing how many rows are returned. It should say 1000.

BUT...

Let's say you forgot and it would be really costly to run the function again, but you still want to know how many staff employees you have. USE THE COUNT FUNCTION!

SELECT count(*) FROM staff 

This returns (you guessed it) 1000. 1000 for the number of employees.

Ahhh. BUT HOW MANY OF THEM ARE WOMEN YOU ASK? NEVER FEAR SQL IS HERE!

SELECT count(*) FROM staff GROUP BY gender

First things first. Gender has to be a column in the table(staff) for you to perform this. It has gender values as Male or Female. Second, GROUP BY is basically saying, yes give me the count from staff, but I want it to be returned in a different way. Count all these employees (hence the star!) but now I want you to count them by their gender and group them!

AHA! We do get returned the right values, BUT!!!! It doesn't tell us which is which! 496 and 504 of what? We can fix this.

SELECT gender, count(*) FROM staff GROUP BY gender

Well why does this fix it. Well remember we wrote in our query to select on the count. In reality, this means return us just the count. When we specify select gender, we are specifically stating to select it with our count as well. Now we know that the number of males are 504 and females are 496.

• Getting the hang of it so far? If you come this far, I would like to recommend this video. VERY efficient video about mysql, but many of the queries are the same. It is amazing. According to someone in the comments, they landed a job by watching that video.

Anyways...let's keep going.

How many employees do we have per deparment?

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(TRY IT YOURSELF FIRST, SEE IF YOU CAN GET IT!)

• ANSWER IS BELOW

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SELECT department, count(*) FROM staff GROUP BY department

Select department! Select their Count as well (for all employees, hence the star!). FROM my staff table inside my database. THESE VALUES SELECTED WILL BE RETURNED! NOW RETURN THEM TO ME BY GROUPING THEM BY THEIR SPECIFIC DEPARTMENT!

Hows that for an all caps explanation? 22 rows for all 22 departments. We are too clever, aren't we?!

What is the highest salary of our employess..?

SELECT max(salary) FROM staff

Yes! Very clever of you. Just change max with min and there you have it. The lowest salary will be returned! Max is just one of the many "math" functions you could run.

Wanna see them at the same time? BUT you also want to see them based on employees in each department? You must be a very efficient person.

SELECT department, min(salary), max(salary)
FROM staff
GROUP BY department

Oh you must have wanted it specific to gender too?

SELECT gender, min(salary), max(salary)
FROM staff
GROUP BY gender

There you have it. Go to your boss and claim you demand a higher salary! (Just kidding!)

Let's move on to statistical functions!

Using statistics.sql file!

How much money is our company spending to pay our employees? Hey! Let's sum up all the salaries in the staff table!

SELECT sum(salary)
FROM staff

97.3 million paid per year across all staff. How much does each department pay in salary?

SELECT department, sum(salary)
FROM staff
GROUP BY department

22 rows for each of our 22 departments! What's the average?!

SELECT department, avg(salary)
FROM staff
GROUP BY department

Variance, Standard Deviations ?

SELECT department, var_pop(salary)
FROM staff
GROUP BY department

SELECT department, stddev_pop(salary)
FROM staff
GROUP BY department

We get way too many decimal places though, so lets round them and say we want 2 decimal places.

SELECT department, round(stddev_pop(salary),2)
FROM staff
GROUP BY department

Using filter_group.sql file!

Filtering our data and grouping data.

Almost like booleans! If your familiar with coding. If not, its like adding clauses to our queries. Greater than, less than, equal to.

Let's see all salaries greater then $100,000, by their name and department they work in.

SELECT last_name, department, salary
FROM staff
WHERE
salary > 100000

We used WHERE function, which is giving a restriction, or a specific request.

OR we might want to see just one department. just add WHERE department = 'Tools' It is important to add the quotation marks, because tools in the database is loaded as a CHAR VAR which is a character variable, commonly known as a string in other coding languages.

If you have 2 restrictions? Just use "AND"

SELECT last_name, department, salary
FROM staff
WHERE
salary > 100000
AND
department = 'Tools'

We want only the department of tools and only the people with high salaries.

We can use OR as well! We will have some people in other departments. We will also have people in tools department with salaries lower than 100000 because we said OR. Give me this ... OR ... give me that.

SELECT last_name, department, salary
FROM staff
WHERE
department = 'Tools'
OR
salary > 100000

ALL DEPARMENTS that begin with letter B and sum of their salaries.

SELECT department, sum(salary)
FROM staff
WHERE
deparment LIKE 'B%'
GROUP BY department

WE used the LIKE function that basically says, WHERE something looks like "this". In our case B. The percent symbol is important here. Putting it before the B will change everything. We want the word to start with B and the % says and of the other letters could be arbitrary, as long as it starts with B, we want it returned.

Beginning with letters Bo..? LIKE 'Bo%'

Baby or Beauty? LIKE 'B%y'

REFORMATTING data.

• file: reformat.sql

We can reformat data to get it into a consisten format. You could change state names into their abbreviations to save space and have easier convenience of use.

SELECT DISTINCT department
FROM staff

DISTINCT allows us to return a specific in our query just once. There are many staff members in Beauty deparment. BUT We do not want Beauty to appear 78 times. We want it to appear once, so we use DISTINCT. Uppercase? Just use UPPER(department)

Job title to combine with department name?

CONCAT IT.

SELECT
job_title || '-' || department
FROM staff

Concatenation is basically adding two things together without spaces in between. Concatenating String and HI = StringHI So we want to separate them with a dash, hence the

|| '-' ||

• We could also give names (titles) to columns we created.

SELECT
job_title || '-' || department title_dept
FROM staff

title_dept becomes the name of our column returned. USE TRIM to remove whitespaces or extra characters.

SELECT
trim('   Sofware Engineer   ')

We cut out the leading and trailing whitespaces. Check it by

SELECT length('   Software Engineer   ') 

with

SELECT length(trim('   Software Engineer   ')) 

Length was reduced! :-)

Let's list all employees with title that begins with word assistant.

SELECT job_title
FROM staff
WHERE job_title like 'Assistant%'

Let's add a boolean, that will replace the WHERE clause

SELECT job_title, (job_title like '%Assistant%') is_asst
FROM staff

I added the first % before Assistant to get all queries where assistant might be the second word. is_asst with return true if assistant is in the table abd false if not.

Extract those strings!

• file: extract_string.sql

SELECT
'abcdefghijkl' test_string

12 character string made. Replace parts of a string with SUBSTRING function.

SELECT
SUBSTRING('abcdefghijkl' FROM 1 FOR 3) test_string

Returns abc. Using FROM 5, starts at 5th letter and prints everything on, inclusively. Index starts at 1, not 0!

SELECT SUBSTRING(job_title FROM 10)
FROM staff
WHERE job_title LIKE 'Assistant%'

We get the words that follow assistant. Because we start from the 10th index.

Want to change the word assistant with Asst?

SELECT OVERLAY(job_title PLACING 'Asst.' FROM 1 for 10)
FROM staff
WHERE job_title LIKE 'Assistant%'

we lost the space in between Asst.Manager. We need to change the length of the string we replace from 10 to 9.

Filtering with regular expressions!

• file: regular_expressions.sql

SELECT job_title
From staff
WHERE job_title Like '%Assistant%'

With this query, we noticed different levels of assistants... Let's select assistants at level 3 or 4. Similar to allows more expressive syntax. Separate III and IV by "OR" character.

SELECT job_title
From staff
WHERE job_title SIMILAR TO '%Assistant%(III|IV)'

Include list of jobs that includes the assistant 4 or 2 characters starting with I.

SELECT job_title
From staff
WHERE job_title SIMILAR TO '%Assistant I_'

Adding the underscore is stating we only want one more character after the I. The only two options are II and IV, which is why these will be the only jobs the query returns.

We can use square brackets as well. Jobs beginning with letters: E, P, OR S.

SELECT job_title
From staff
WHERE job_title SIMILAR TO '[EPS]%'

Reformatting Numeric Data!

• file: reformat_number.sql

SELECT department, avg(salary)
FROM staff
GROUP BY department

Avg salary produces more digits than necessary! We could address this by using truncate function TRUNC. Does not round. It just ignores Decimals!

SELECT department, trunc(avg(salary))
FROM staff
GROUP BY department

Want to return the next largest integer? Use ceiling function ceil(avg(salary)).

SUBQUERIES in SELECT CLAUSES.

• Sub Queries can be used in three different parts of a select statement. In the list of values returned, in the From Clause and in the Where Clause.

SELECT
last_name, salary, department
FROM staff

Now, because we'll have multiple select statements within a single query we'll want to make sure the database can tell which table each value comes from. So to do this, we'll use a table alias and include that alias as a prefix for each value we'll return.

SELECT
s1.last_name, s1.salary, s1.department
FROM staff s1

Now, let's compare the salarys with the avg salary. We will use a nested select operator, as I like to call it.

SELECT
s1.last_name, s1.salary, s1.department, (SELECT round(avg(salary)) FROM staff s2 WHERE s2.department = s1.department)
FROM staff s1

Let me explain. Our inner query, calculated avg salary, but only uses rows where department is equal to department of employee we are currently looking at. You can see this adds an extra column.

• We use a where clause that references a table in the top level query so that the subquery knows which row is referenced!

SUBQUERIES in FROM CLAUSES.

• file: continuation from previous.

salary > 100000 is executive, by our assumption.

We would like to find the average executive salary by department.

SELECT s1.department, round(avg(s1.salary))
FROM
  (SELECT department, salary 
  FROM staff
  WHERE salary > 100000) s1
GROUP BY s1.department

The inner SELECT is basically telling our outer SELECT to only select department and avg salary from the values that are returned from our inner select. (AFTER our FROM clause).

SUBQUERIES in WHERE CLAUSES. Useful for Comparisons within a single table.

WHEN USING SUBQUERIES use an alias, like I used previously with s1 and s2.

Find department of person with highest salary.

SELECT s1.department, s1.last_name, s1.salary
FROM staff s1
WHERE 
s1.salary = (SELECT max(s2.salary) FROM staff s2) #( This subquery- returns max salary)

Stanley, in the grocery deparment, making good money!

JOINING THOSE TABLES!

• file: joins.sql
• When working with SQL, we will sometimes need to retrieve data from multiple tables. For example, the Staff table includes a department for each employee. Departments are organized into divisions. Since we don't keep division information in the staff table, we have to look it up somewhere else.

SELECT * FROM company_divisions  

21 rows returned. Table has 2 columns. Departments and division. Let's join staff and company_division.

SELECT s.last_name, s_department,cd.company_division
FROM staff s
JOIN company_divisions cd
ON s.department = cd.department

Returned 953 rows, but we should have 1000. missing 47... mhm.

We use an outer join to return all rows. LEFT outer joins or right outer joins which depends on ordering of table.

SELECT s.last_name, s_department,cd.company_division
FROM staff s LEFT JOIN company_divisions cd
ON s.department = cd.department

Again, by left joining we're going to take all of the rows in the Staff table, even if there isn't a corresponding row in the Company Divisions table.

SELECT s.last_name, s_department,cd.company_division
FROM staff s LEFT JOIN company_divisions cd
ON s.department = cd.department
WHERE cd.company_division is NULL

They all come from the books department.

Creating a View.

• file: groupings.sql

SELECT s.*, cd.company_division, cr.company_regions
FROM staff s
LEFT JOIN company_divisions cd
ON s.department = cd.department
LEFT JOIN
company_regions cr
ON s.region_id = cr.region_id

What we have here is a select statement that uses two left joins and it selects all the rows from the staff table, it selects the company division, and the company regions name. It returns it as a single table. Rather than retyping this if we want to see it, we create a view.

CREATE VIEW staff_div_reg AS
SELECT s.*, cd.company_division, cr.company_regions
FROM staff s
LEFT JOIN company_divisions cd
ON s.department = cd.department
LEFT JOIN
company_regions cr
ON s.region_id = cr.region_id

Want to view it?

SELECT count(*)
FROM staff_div_reg

We get a 1000. Check.

Grouping and Totaling.

SELECT company_region, count(*)
FROM staff_div_reg
GROUP BY company_region
ORDER BY company_region # so we get it alphabetically.

If we want counts by both region and division. Use grouping sets.

SELECT company_division, company_region, count(*)
FROM staff_div_reg
GROUP BY 
GROUPING SETS (company_division,company_region)
ORDER BY company_region, company_division # so we get it alphabetically.

We also get totals by company division. We also get 47, with no name. That was the books department. It has no division. Let's add gender.

SELECT company_division, company_region,gender,count(*)
FROM staff_div_reg
GROUP BY 
GROUPING SETS (company_division, company_region, gender)
ORDER BY company_region, company_division, gender # so we get it alphabetically.

WAYS TO GROUP DATA: ROLLUPS AND CUBES.

• file: rollup-cube

Now first, let's modify the staff division region view we created to include country code. To do that, I'm going to use the command CREATE OR REPLACE VIEW, and as the name implies, if this view doesn't exist, it will simply create it. If there is a version of this view that already exists, it'll replace that version with the version I'm about to specify.

CREATE OR REPLACE VIEW staff_div_reg_country AS
SELECT s.*, cd.company_division, cr.company_regions, cr.country
FROM staff s
LEFT JOIN company_divisions cd
ON s.department = cd.department
LEFT JOIN company_regions cr
ON s.region_id = cr.region_id

Let's select num of employees by company, region, country.

SELECT company_regions, country, count(*)
FROM staff_div_reg_country
GROUP BY company_regions, country
ORDER BY country, company_regions

Want totals for each country, use rollup in group by clause.

SELECT company_regions, country, count(*)
FROM staff_div_reg_country
GROUP BY 
ROLLUP(country,company_regions)
ORDER BY country, company_regions

Now, for more advanced breakdowns, we can use the CUBE operation on the GROUP BY clause. This tells SQL to create all possible combinations of sets of grouping columns. For example, for each division, show results by region. So we will move the ORDER BY here and we'll remove ROLLUP, and we'll specify CUBE, and we're going to use not country, but we're going to use a cube of company_division.

SELECT company_division, company_regions, count(*)
FROM staff_div_reg_country
GROUP BY 
CUBE(company_division, company_regions)

Your cube needs to match your select for table names.

FETCH FIRST.

• file: fetch_first.sql

Working with large data sets, we might want to list employees with top ten salaries. Now, what I'm going to do is add a clause called fetch first. Now, fetch first works with the order by clause to sort and limit results. Fetch first is like the limit keyword, in that only a fixed number of rows are returned, but with fetch first, the ordering is performed before choosing the rows to return. So, I'll specify fetch first, 10 rows only, so this will return only 10 rows.

SELECT last_name, job_title, salary
FROM staff
ORDER BY salary DESC #DESC is return values in descending order
FETCH FIRST
10 ROWS ONLY

LIMIT would limit the rows and then perform operations (DESC), which is not what we want.

Let's build aggregation query.

SELECT company_division, count(*)
FROM staff_div_reg_country
GROUP BY company_division
ORDER BY count(*)

Let's turn this into descending order

SELECT company_division, count(*)
FROM staff_div_reg_country
GROUP BY company_division
ORDER BY count(*) DESC
FETCH FIRST
5 ROWS ONLY

Window Functions and Ordered Data.

• file: window.sql

Window functions allow us to make SQL statements about rows that are related to the current row during processing. This is somewhat like the way subqueries work. They let us do an operation that's related to the current row that SQL is processing. For example, instead of using a subquery to calculate an average salary for an employee's department, we can use a windowing function on rows called OVER PARTITION.

SELECT department, last_name, salary, avg(salary) OVER (PARTITION BY department)
FROM staff

So again, what I have is I've selected department, last_name, and salary, and I'm going to also display an average of salary for each department, and that's what the OVER PARTITION BY statement does.

SELECT company_region, last_name, salary, min(salary) OVER (PARTITION BY company_region)
FROM staff_div_reg

Min salary that is earned by based on region, not department. :-)

Window Functions.

We could also select a set of attributes grouped by department and include the first value by department in each row using something called The First Value Function.

SELECT department, last_name, salary,
first_value(salary) OVER (PARTITION BY department ORDER BY salary DESC)
FROM staff

I want the first value in the list of salaries, where its paritioned by department and in descending salary order. In this case, first_value is acting like a max value, its the first value in descending salary list.

We could order by last name.

SELECT department, last_name, salary,
first_value(salary) OVER (PARTITION BY department ORDER BY last_name)
FROM staff

first_value is the salary of the first person in the table, after names are in alphabetical order.

Window Functions: Rank.

It works with parition function to order results and assing a rank value based on the way paritition data is sorted.

SELECT department, last_name, salary,
rank() OVER (PARTITION BY department ORDER BY salary DESC)
FROM staff

Salaries are sorted and rank lists a number from 1 to the number of employees in the department. Ranking restarts by department and ranks by salary.

LAG and LEAD.

• file: lead_lag.sql

We can reference rows RELATIVE to the currently processed rows.

SELECT department, last_name, salary,
lag(salary) OVER (PARTITION BY department ORDER BY salary DESC)
FROM staff

lag refers to the row that came before a currently processed row.

SELECT department, last_name, salary,
lead(salary) OVER (PARTITION BY department ORDER BY salary DESC)
FROM staff

Lead will show rows that came after a currently processed row.

NTILE functions.

Sometimes we want to group rows into some number of buckets or ordered groups. We can use the ntiles function to assign buckets to rows. This allows us to easily calculate statistics like quartiles over sets of rows.

Ntile takes a number, and it will be the number of buckets that we want or ordered groups that we want

SELECT department, last_name, salary
ntile(4) OVER (PARTITION BY department ORDER BY salary DESC)
FROM staff

Top earners are assigned value 1. Etc. ntile resets with each department.

That's all!

Pat yourself on the back. Give yourself a hug. Whatever suits you!

Thanks for following along. You have now learned ALOT. Remember:

1. Don't underestimate time needed to collect and prepare data.
2. Remember that an inner jointed SQL will return rows only when both tables have corresponding rows. If you want all the rows from one table even if there is not a corresponding row in the other table, then use an outer join.
3. Cubes, rollups, and grouping sets are useful when you need to produce cross-tabulations and subtotals.
4. Window Functions help us focus on sets of related rows such as all rows in a single department or company region. Window functions can help simplify select statements that would otherwise require subqueries.
5. SQL statements get complicated especially when using joins, subqueries, and complicated grouping and filtering clauses. Use views to capture this logic so that you can query the view instead of having to repeatedly type in long SQL statements. It's easy to make a mistake when typing in such statements so use views that you can test and verify.
6. Enjoy coding!