Hello as the title suggest I need help writing a query that does this. I need to find all the users who have had a badge number change. So in the database there are often two records for the same person but both have a different badge number. Im assuming it's the same person if the social matches.
Table:
Badge_no | SSN
123123 | 387-47-1234 2
34837 | 387-47-1234
837532 | 543-45-6392
584391 | 543-45-6392
In this case I would want it to output:
837532 | 543-45-6392
584391 | 543-45-6392
Thank you!
I believe the following should do the trick here:
SELECT *
FROM yourtable
WHERE SSN IN (SELECT SSN FROM yourtable GROUP BY SSN HAVING Count(*) >=2);
That subquery will return SSN's that have more than one record. We use those SSN's to select, again, from the table to get all of the fields associated to them.
Related
I have tried searching the web and whilst there are plenty of answers for finding duplicates, I am yet to stumble on one that allows me to find all the duplicates within a column (i.e where the same 'name' occurs more than once) and then only select the lowest row id (which would be the first duplicate name entered).
So the table's description (inserted from a file):
create table customer(id int, name varchar,)
id| name
1 | Darren
2 | Mark
3 | Julie
4 | Mark
5 | Julie
The query:
CREATE VIEW AS
SELECT COUNT(name), name
FROM customer
GROUP BY name
HAVING COUNT(name) > 1
Result (the order is never guaranteed, I want Mark to always come first as he has the lowest id):
Julie
Mark
Now the issue is, if i select id I have to include it in the group by. Doing that means no duplicate columns get selected as there wont be any since ever id is unique. And without selecting id I cant ORDER BY desc.
I hope I am clear, if not I can re-word or supply more information.
Please try this? Nested query. Basically the SELECT/GROUP is called. On the outside, we get the information selected and sort it.
CREATE VIEW AS
SELECT CNT_NAME, NAME
FROM
(
SELECT COUNT(name) CNT_NAME, name, min(id) min_id
FROM customer
GROUP BY name
HAVING COUNT(name) > 1
) AS alias
ORDER BY MIN_ID
Looking around in the (now-discontinued) documentation and found this example:
Subquery in FROM clause
A subquery in a FROM clause acts similarly to a temporary table that is generated during the execution of a query and lost afterwards.
SELECT Managers.Id, Employees.Salary
FROM (
SELECT Id
FROM Employees
WHERE ManagerId IS NULL
) AS Managers
JOIN Employees ON Managers.Id = Employees.Id
(Excerpted from Subqueries - Subquery in FROM clause. The original author was Phrancis. Attribution details can be found on the contributor page. The source is licenced under CC BY-SA 3.0 and may be found in the Documentation archive. Reference topic ID: 1030 and example ID: 3327.)
My question is:
why using an extra ManagerId. An Id column is already in
Employees table,
why have the extra ManagerId to be null for a manager (ok it wants to be a joke).
My opinion:
despite the upvotes, something is wrong with this is example,
Tables with example data would be nice to see on the fly how it's working. One table with start data, one table temporary SELECT and one table the
resultset.
Edit: Thanks to all contributors for their answers!
#Alex K.: That is my point of view "it is not something one would actually use". But people, who wants to learn SQL, might think, that it is good practice, because it is in the documentation here.,
#Nebi: Thanks for the point that one would write it simpler to get the same result.
#Unnikrishnan R: "showcase how the sub query works" does in my eyes not only mean that it is fully functional but additional that it makes sense. If I get things simpler, why doing it the errorprone hard way.
#me: should have titled it "let's discuss sql documentation" or like that ;)
Let us consider a situation where Employee table holds all employees including their managers in which employee has an Id, and there is also a column for the manager Id (which can be null). This can be the point of view ,who was writing that SQL queries.
For Example,
+----+-------+--------+-----------+
| Id | Name | Salary | ManagerId |
+----+-------+--------+-----------+
| 1 | Joe | 70000 | 3 |
| 2 | Henry | 80000 | 4 |
| 3 | Sam | 60000 | NULL |
| 4 | Max | 90000 | NULL |
+----+-------+--------+-----------+
why have the extra ManagerId to be null for a manager --
getting the employees that are not managers
It is just an example how to do/ use Subqueries.
To your questions:
why using an extra ManagerId. An Id column is already in Employees
table
First of all ManagerId and Id are different columns of the table Employees. So there is a difference between them. But you might reffering to the Id of the Subquery Managers and the Id of the joined table Employees.
Then you need to define which Id you are using. Else you get the Error for ambigiuos column. In this example you to specify either the Subqueries Id which is Managers.Id or the Id of the joined table Employees (Employees.Id). Which one you choose is totally regardless because you use INNER JOIN one the Id.
why have the extra ManagerId to be null for a manager (ok it wants
to be a joke).
This is because of getting all the Employees that have are not managers. You are right about saying this could be done easier or in other form. For instance:
SELECT Id, Salary
FROM Employees
WHERE ManagerId IS NULL
This probably gets the same result as in the original. But the example is not about that, it is about the structure of a subquery.
why using an extra ManagerId. An Id column is already in Employees table
Consider you are having an employee table and you also wanted to keep the manager information in the same table.so apart from the ID column you need to add another column to keep the managerid.
why have the extra ManagerId to be null for a manager (ok it wants to be a joke).
The query is just to showcase how the sub query works. In this case subquery retrieves the manager from the Employee table (managerID is null) then join those id's with Employee table in the outer query to get the salary of each managers.
I'm trying to count staff that belong to certain branch in sql, so branch X has Y number of employees for example. There are two tables I need to query number 1. and 2. below. I want to achieve the following:branchno | number of staff in that branch
The 'Staff' table: staffno | fname | lname | position | sex | dob | salary | branchno
the 'branch' table: branchno | street | city | postcode
Thank you kindly
This is a really basic query, and if you have problems solving it, just break it down step by step:
What is it you want to get? A count of people in a certain branch. How do you choose what information to display? With a selectstatement.
What do you want to do with the information before you show it? You want to count it with a count()aggregate function.
Where do you get the data from? A table, selected with a fromstatement.
And finally, you only want from a specific branch so what do you do? You filter the results with a whereclause.
And putting it all together you get this:
SELECT COUNT(staffno) FROM Staff WHERE branchno = 1
If you want the branch with number 1.
Edit: I just noticed the comment you made about branches without employees, and taking that into account you need the branch table too.
SELECT b.branchno, COUNT(staffno) AS StaffCount FROM branch b
LEFT JOIN staff s ON b.branchno = s.branchno
-- WHERE b.branchno = 1 -- optional filter
GROUP BY b.branchno
Using a left joinbetween the tables means that you'll get all rows from branch and the matching rows from staff.
I've a table like this one:
Column | Type | Modifiers
username | character varying(12) | not null
electioncode | integer | not null
votes | integer | default 0
PRIMARY KEY (username, electioncode)
i need to create a view with username, electioncode, max(votes)
if i use this query it works fine but without username:
SELECT electioncode, max(votes) from table group by electioncode;
if i add username it asks me to add it into the group by but if i do that it gives me the entire table instead of just the username-electioncode-maxvotes
Do you want to get username associated with this number of votes? Or any username in given election code?
If the first:
SELECT
DISTINCT ON ( electioncode )
*
FROM table
ORDER BY electioncode, votes desc;
if the other:
SELECT
electioncode,
min(username),
max(votes)
FROM
table
GROUP BY electioncode;
Your username field seems to be unique. Every record has different username (I am assuming) thus when you group by username it will give you all the records. What you are trying to do has a logic issue not syntax issue.
A suggestion: You want to write on a piece of paper the output you would like to see and then construct the query... If you want Username, Electioncode and max (votes) then imagine how you would display the data where two usernames - user1 and user 2 who have electioncode 001 and voted 1 each? How would you display this?
CREATE TABLE `names` ( `name` varchar(20) );
Assume the names table contains all 40 million first names of everyone living in California (for example).
SELECT count(*) as count, name FROM names GROUP BY name ORDER BY name;
How can I optimize this query?
Expected Result:
count | name
9999 | joe
9995 | mike
9990 | kate
.... | ....
2 | kal-el
You have to create an index on the name column of your table. The query is as good as it can be.
Well, what makes you think it's not already optimised? This looks like the sort of query a good database engine should be able to handle relatively easily - particularly if you've got an appropriate index on your table.
Do you actually have a bottleneck here, or are you worrying about something that might happen in the future? If it's the latter, I suggest you try it with your RDBMS (by generating dummy data), and see what happens.