Display categories and their counts - mysql

I have two tables in my database (Jobs and Categories), I need help with an SQL that will select the categories and also count the number of jobs in each category. I am using MySQL if that helps.
So far
SELECT * from categories c left join jobs j on (c.job_id = j.job_id);
Thanks

Assuming you have a CategoryName column...
select c.CategoryName,
count(*)
from Categories c
join Jobs j on j.job_id = c.jobID
group by c.CategoryName

Assuming your table structure is similar to this:
Category
ID, Name
Job
ID, CategoryID, Name
SELECT Category.Name, COUNT(Job.ID)
FROM Job
INNER JOIN Category ON Job.CategoryID = Category.ID

Related

SQL Query to see all productName and which category its belong

So I have a task all I need to do is to write Query to
select all the product names and which category they belong to.
I Using the tables from W3SCHOOL - https://www.w3schools.com/sql/trysql.asp?filename=trysql_select_all
I Would like to know why this Query -
SELECT p.ProductName
, c.CategoryName
From Products p
Join Categories c
ON p.ProductID = c.CategoryID
not working for me, it gives me only 8 Products with 8 different category names and I need all of the products name from each category.
thank you.
Your query is almost correct, you are joining on the wrong column,
SELECT Products.ProductName, Categories.CategoryName
FROM Products
JOIN Categories ON Categories.CategoryID = Products.CategoryID
should do.
The issue with your join query
You are joining Products.ProductID when you should join Products.CategoryID
The correct query should be as the following:
SELECT p.ProductName, c.CategoryName
FROM Products p
JOIN Categories c ON p.CategoryID = c.CategoryID

Find the count of categories from a table

I am trying to develop an enquiry management system for an institute.
I have 3 tables in mysql db.
leads_course_category table keeps course categories and has the columns: category_id, category_name, category_created_time, category_deleted.
leads_course_details table stores courses under categories and has the columns: course_id, course_category_id, course_name.
leads_enquiry_details table stores enquiry details and has the columns: enquiry_id, enquiry_name, leads_course_details_course_id, enquiry_deleted
I need to find the number of enquiries for each category.
I tried the following query:
SELECT category_name,COUNT(*) as COUNT
FROM leads_course_category
RIGHT JOIN leads_course_details
on category_id=leads_course_details.course_category_id
RIGHT JOIN `leads_enquiry_details`
on leads_course_details.course_id= leads_enquiry_details.leads_course_details_course_id
WHERE leads_enquiry_details.enquiry_deleted=1
GROUP BY leads_course_category.category_id
This query skips all the categories having null values, but I need to show that as count of zero.
Please help me to resolve this.
The condition enquiry_deleted = 1 must be moved to the ON clause:
select c.category_name, count(ed.enquiry_id) count
from leads_course_category c
left join leads_course_details cd on c.category_id = cd.course_category_id
left join leads_enquiry_details ed on cd.course_id = ed.leads_course_details_course_id and ed.enquiry_deleted = 1
group by c.category_id, c.category_name

mysql join on product category multiple relation query

I have three tables as below, each product may belong to multiple categories.
PRODUCT TABLE AS P
1. ID
2. NAME
CATEGORY TABLE AS C
1. ID
2. NAME
RELATION TABLE AS R
1. ID
2. P_ID
3. C_ID
Now I want to get a list of all products in product table, and with their belonging category name display as well.
How do I write this query?
I can get the category ids pulled from the same query, but don't know how to get all the name pulled as well. Here's what I tried.
select p.*,y.*
from p
left join (select p_id,group_concat(c_id) as category_ids
from relation group by p_id) as y on p.id=y.p_id
Do both JOIN operations (to the relation table, and from there to the table containing the category names) and feed the result to your aggregation function (GROUP_CONCAT)
SELECT P.Name, GROUP_CONCAT(DISTINCT C.Name ORDER BY C.Name SEPARATOR '|') categories
FROM Product P
LEFT JOIN Relation R ON P.ID = R.P_ID
LEFT JOIN Category C ON R.C_ID = C.ID
GROUP BY P.ID, P.Name
This will give you one row per product with the categories separated by |.
This uses LEFT JOIN operations so it won't suppress products that have no categories.
Select P.Name, C.Name
From RELATION R Inner Join PRODUCT P
On R.P_ID=P.Id
Inner Join Category C
On C.Id=R.C_ID
This query will get you all the products, with their corresponding category.
I want to give you a small explanation about the difference between Inner Join and Left Join.
If we take as an example 2 tables :
TA(idA, description) and TB(idB, idA, description).
Select TA.description, TB.description
From TA Inner Join TB On TA.IdA = TB.IdA
will get only the rows in TA, that have a corresponding one in TB.
On the other side,
Select TA.description, TB.description
From TA Left Join TB On TA.IdA = TB.IdA
will get all the rows of TA and if the row in TA doesn't have a corresponding one in TB, TB.description for this row will be NULL.
Hope this helps!

SQL Join involving 3 tables, how to?

SQL newbie here.
So we have 3 tables:
categories(cat_id,name);
products(prod_id,name);
relationships(prod_id,cat_id);
It is a one-to-many relationship.
So, given a category name say "Books". How do I find all the products that come under books?
As an example,
categories(1,Books);
categories(2,Phones);
products(302,Sherlock Holmes);
relationships(302,1);
You need to JOIN the three tables.
SELECT p.*
FROM relationships r
INNER JOIN products p
ON p.prod_id = r.prod_id
INNER JOIN categories c
ON c.cat_d = r.cat_id
WHERE c.name = 'Books'
You have to join tables on related columns and specify WHERE clause to select all records where category name = 'Books'
SELECT p.*
FROM categories c
JOIN relationships r ON c.cat_id = r.cat_id
JOIN products p ON r.prod_id = p.prod_id
WHERE c.name = 'Books' -- or specify parameter like #Books
In SQL you often join related tables and beginners tend to join, whatever the situation. I would not recommend this. In your case you want to select products. If you only want to show products data, select from products only. You want to select products that are in the category 'Books' (or for which exists an entry in category 'Books'). Hence use an IN or EXISTS clause in order to find them:
select * from products
where prod_id in
(
select prod_id
from relationships
where cat_id = (select cat_id from categories where name = 'Books')
);
Thus you get a well structured query that tells the reader easily how the tables are related and what data you are actually interested in. Later, with different tables and data to select, this may keep you from duplicate result rows that you must get rid of by using DISTINCT or from getting wrong aggregates (sums, counts, etc.), because of mistakenly considering records multifold.
try this:
select p.Prod_id,p.name
from products p inner join relationships r on
p.prod_id = r.prod_id
where r.cat_id = (select cat_id from categories where name = 'books')
or
select p.Prod_id,p.name
from products p inner join relationships r on
p.prod_id = r.prod_id inner join categories c on c.cat_id = r.cat_id
where c.name = 'books'

Need help combining two functional mysql querys into one?

I have this query that will return a list of all of the people associated with Thomas and their ids.
SELECT c.name, c.ID
FROM namesandID s, associations o, namesandID c
WHERE s.name='Thomas' AND o.id = s.ID AND o.associateID = c.ID
GROUP BY c.ID;
Then I have this query that I can manually type in the id number and it will return the correct count of associates.
SELECT count(*) FROM (
SELECT associateID FROM associations WHERE id=18827 GROUP BY associateID
) AS t;
My goal is to have one query that will take Thomas as the name and return three columns that will have his associate their id number an the number of people they are associated with.
Also as some additional information this is a very large database with about 4million rows and 300million associations so any speed increase on either of these queries would be greatly welcomed.
Not tested, however the below should work:
select
c.name,
c.id,
assoc_count.cnt
from
namesandIds s
inner join
associations o on
o.id = s.ID
inner join
namesandId c on
c.ID = o.associateId
left outer join
(
select
id,
count(distinct associateId) as cnt
from
associations
group by
id
) assoc_count on
assoc_count.id = c.ID
where
s.name = 'Thomas'
Not very efficient but
SELECT c.name, c.ID, COUNT(DISTINCT o.associateID)
FROM {the rest of the first query}
should do the trick.