sql server - how to modify values in a query statement? - sql

I have a statement like this:
select lastname,firstname,email,floorid
from employee
where locationid=1
and (statusid=1 or statusid=3)
order by floorid,lastname,firstname,email
The problem is the column floorid. The result of this query is showing the id of the floors.
There is this table called floor (has like 30 rows), which has columns id and floornumber. The floorid (in above statement) values match the id of the table floor.
I want the above query to switch the floorid values into the associated values of the floornumber column in the floor table.
Can anyone show me how to do this please?
I am using Microsoft sql server 2008 r2.
I am new to sql and I need a clear and understandable method if possible.


select lastname,
firstname,
email,
floor.floornumber
from employee
inner join floor on floor.id = employee.floorid
where locationid = 1
and (statusid = 1 or statusid = 3)
order by floorid, lastname, firstname, email


You have to do a simple join where you check, if the floorid matches the id of your floor table. Then you use the floornumber of the table floor.
select a.lastname,a.firstname,a.email,b.floornumber
from employee a
join floor b on a.floorid = b.id
where a.locationid=1 and (a.statusid=1 or a.statusid=3)
order by a.floorid,a.lastname,a.firstname,a.email


You need to use a join.
This will join the two tables on a certain field.
This way you can SELECTcolumns from more than one table at the time.
When you join two tables you have to specify on which column you want to join them.
In your example, you'd have to do this:
from employee join floor on employee.floorid = floor.id
Since you are new to SQL you must know a few things. With the other enaswers you have on this question, people use aliases instead of repeating the table name.
from employee a join floor b
means that from now on the table employee will be known as a and the table floor as b. This is really usefull when you have a lot of joins to do.
Now let's say both table have a column name. In your select you have to say from which table you want to pick the column name. If you only write this
SELECT name from Employee a join floor b on a.id = b.id
the compiler won't understand from which table you want to get the column name. You would have to specify it like this :
SELECT Employee.name from Employee a join floor b on a.id = b.id or if you prefer with aliases :
SELECT a.name from Employee a join floor b on a.id = b.id
Finally there are many type of joins.
Inner join ( what you are using because simply typing Join will refer to an inner join.
Left outer join
Right outer join
Self join
...
To should refer to this article about joins to know how to use them correctly.
Hope this helps.

Related

how to do a left join with no duplicate columns?

I have to join 2 tables and the first table I'm joining consists of:
Physicians (ID, FirstName, LastName, PracticeID, SpecialtyID, Email)
and the second table I have is:
PhysicianSpecialties( SpecialtyID, SpecialtyName)
I wrote this query to join the tables together
Select *
from physicians
right join PhysicianSpecialities
on PhysicianSpecialities.SpecialtyID = Physicians.SpecialtyID
and when I left Join them the table is now
(ID, FirstName, LastName, PracticeID, SpecialtyID, Email, SpecialtyID, SpecialtyName)
how can I rewrite this so there is only one "SpecialtyID" Column?

You didn't specify the DBMS product you are using, but: in standard SQL you can join with the USING operator if the join columns have the same name in both tables.
In this case, the "duplicated" column will automatically be removed from the result.
Select *
from physicians
right join PhysicianSpecialities using (SpecialtyID)
Not all DBMS products support that though.

You need to specify the column names of both tables instead of (*)
Select a.*,b.SpecialtyName
from physicians a
right join PhysicianSpecialities b
on b.SpecialtyID = a.SpecialtyID

Use
SELECT
physicians.ID, physicians.FirstName, physicians.LastName,
physicians.PracticeID, physicians.SpecialtyID, physicians.Email,
PhysicianSpecialities.SpecialtyName
instead of SELECT *, hence your query goes like:
SELECT
physicians.ID, physicians.FirstName, physicians.LastName,
physicians.PracticeID, physicians.SpecialtyID, physicians.Email,
PhysicianSpecialities.SpecialtyName
FROM
physicians
LEFT JOIN
PhysicianSpecialities ON Physicians.SpecialtyID = PhysicianSpecialities.SpecialtyID;
I hope it will return the desired result.

sql join tables IS NOT

I need to get the data of all teachers who gives a course but does not have an assignement. I tried using inner or left join so if the name of the teacher is not in the table of the assignments it will show the data. If the name of the teacher is in the table of assignments it must not show the data. But I could not get it working.
What am I doing wrong?
select teachers.nameteacher, courses.namecourse, courses.codecourse
from teachers
inner join courses
on teachers.codecourse = courses.codecourse
left join assignments
on assignments.nameteacher = teachers.nameteacher
where teachers.nameteacher IS NULL

filter should be from assignments table
SELECT t.nameteacher,
c.namecourse,
c.codecourse
FROM teachers t
INNER JOIN courses c
ON t.codecourse = c.codecourse
LEFT JOIN assignments a
ON a.nameteacher = t.nameteacher
WHERE a.nameteacher IS NULL --should be assignments
Note : start using alias names to make the query more readable

The easy (and efficient) way, if you are using PostgreSQL or Microsoft SQL Server is to say:
select
a.nameteacher,
c.namecourse,
c.codecourse
from
(
select nameteacher from teachers
except
select nameteacher from assignments
) a
join teachers b
on a.nameteacher = b.nameteacher
join courses c
on b.codecourse = c.codecourse;
If you are using Oracle change "except" to "minus".
If you are using SQLite or MySQL you need to use one of the other suggestions because these don't support except or minus.

I want to retrieve data from 4 tables in SQL Server

I want to retrieve data from 4 tables. Patient table has id as PK which is the foreign key in other three tables ett, phar and ssc. Where a patient lie in only one category. i.e patient id pt1 exists in either of the 3 tables. now I want to retrieve patient info along with its associated category.
My query is:
SELECT *
FROM Patient p
INNER JOIN ETT t
ON p.Patient_ID = t.Patient_ID || INNER JOIN Pharmacological ph
ON p.Patient_ID = ph.Patient_ID
I used OR clause because I want only 1 inner join executing at one time. but its not giving me results, any suggestions??

....Patient table has ID as PK which is the foreign key in other three
tables name: ett, phar and ssc where a patient lie in only one
category. Example, patient id pt1 exists in either of the 3 tables.
Based on your statement, you can join all the tables in table Patient using LEFT JOIN since a record can only exist on one table. The query below uses COALESCE which returns the first non-null value with int the list.
The only thing you need is to manually specify the column names that you want to be shown on the list as shown below.
SELECT a.*,
COALESCE(t.colA, p.ColA, s.ColA) ColA,
COALESCE(t.colB, p.ColB, s.ColB) ColB,
COALESCE(t.colN, p.ColN, s.ColN) ColN
FROM Patient a
LEFT JOIN ETT t
ON a.Patient_ID = t.Patient_ID
LEFT JOIN Phar p
ON a.Patient_ID = p.Patient_ID
LEFT JOIN SSC s
ON a.Patient_ID = s.Patient_ID
To further gain more knowledge about joins, kindly visit the link below:
Visual Representation of SQL Joins

For or - do not ise ||, use "or"
You cannot join with or, you need re-format your query.

Oracle/plsq Inner join multiple column to table one column

I have problems joining 2 table let say I have a column on the 1st table name nationalitycode(this is number) and it fetches the description(it could be american, chinese etc) depending on the first table nationalitycode. So I tried inner join the table so the first inner join is good and working perfectly.
here is the code:
SELECT person.firstnm, person.middlenm, person.lastnm, refcd.description
FROM person
INNER JOIN refcd
ON person.natcd = refcd.id;
However when I try to join the 2nd column from the 1st table let say the column name is gencd(gendercode its a foreign key) but when I updated the query to inner join 2 column from one table getting the value from one column in the 2nd table I get this error.
"Column ambigously defined" I get this error and I understand this my question is is there anyway to achieve my desired output? I want to join 2 table, specifically joining 2 column on table 1 to 1 column in table 2.
Here is my updated query:
SELECT person.firstnm, person.middlenm, person.lastnm, refcd.description
FROM person
INNER JOIN refcd
ON person.natcd = refcd.id
INNER JOIN refcd
ON person.gencd = refcd.id;
Help is really appreciated.

Your query should be
SELECT person.firstnm, person.middlenm, person.lastnm, refcd.description
FROM person INNER JOIN refcd ON person.natcd = refcd.id
INNER JOIN refcd R2 ON person.gencd = R2.id;

Explanation of self-joins

I don't understand the need for self-joins. Can someone please explain them to me?
A simple example would be very helpful.

You can view self-join as two identical tables. But in normalization, you cannot create two copies of the table so you just simulate having two tables with self-join.
Suppose you have two tables:
Table emp1
Id Name Boss_id
1 ABC 3
2 DEF 1
3 XYZ 2
Table emp2
Id Name Boss_id
1 ABC 3
2 DEF 1
3 XYZ 2
Now, if you want to get the name of each employee with his or her boss' names:
select c1.Name , c2.Name As Boss
from emp1 c1
inner join emp2 c2 on c1.Boss_id = c2.Id
Which will output the following table:
Name Boss
ABC XYZ
DEF ABC
XYZ DEF

It's quite common when you have a table that references itself. Example: an employee table where every employee can have a manager, and you want to list all employees and the name of their manager.
SELECT e.name, m.name
FROM employees e LEFT OUTER JOIN employees m
ON e.manager = m.id

A self join is a join of a table with itself.
A common use case is when the table stores entities (records) which have a hierarchical relationship between them. For example a table containing person information (Name, DOB, Address...) and including a column where the ID of the Father (and/or of the mother) is included. Then with a small query like
SELECT Child.ID, Child.Name, Child.PhoneNumber, Father.Name, Father.PhoneNumber
FROM myTableOfPersons As Child
LEFT OUTER JOIN myTableOfPersons As Father ON Child.FatherId = Father.ID
WHERE Child.City = 'Chicago' -- Or some other condition or none
we can get info about both child and father (and mother, with a second self join etc. and even grand parents etc...) in the same query.

Let's say you have a table users, set up like so:
user ID
user name
user's manager's ID
In this situation, if you wanted to pull out both the user's information and the manager's information in one query, you might do this:
SELECT users.user_id, users.user_name, managers.user_id AS manager_id, managers.user_name AS manager_name INNER JOIN users AS manager ON users.manager_id=manager.user_id

Imagine a table called Employee as described below. All employees have a manager which is also an employee (maybe except for the CEO, whose manager_id would be null)
Table (Employee):
int id,
varchar name,
int manager_id
You could then use the following select to find all employees and their managers:
select e1.name, e2.name as ManagerName
from Employee e1, Employee e2 where
where e1.manager_id = e2.id

They are useful if your table is self-referential. For example, for a table of pages, each page may have a next and previous link. These would be the IDs of other pages in the same table. If at some point you want to get a triple of successive pages, you'd do two self-joins on the next and previous columns with the same table's id column.

Without the ability for a table to reference itself, we'd have to create as many tables for hierarchy levels as the number of layers in the hierarchy. But since that functionality is available, you join the table to itself and sql treats it as two separate tables, so everything is stored nicely in one place.

Apart from the answers mentioned above (which are very well explained), I would like to add one example so that the use of Self Join can be easily shown.
Suppose you have a table named CUSTOMERS which has the following attributes:
CustomerID, CustomerName, ContactName, City, Country.
Now you want to list all those who are from the "same city" .
You will have to think of a replica of this table so that we can join them on the basis of CITY. The query below will clearly show what it means:
SELECT A.CustomerName AS CustomerName1, B.CustomerName AS CustomerName2,
A.City
FROM Customers A, Customers B
WHERE A.CustomerID <> B.CustomerID
AND A.City = B.City
ORDER BY A.City;

There are many correct answers here, but there is a variation that is equally correct. You can place your join conditions in the join statement instead of the WHERE clause.
SELECT e1.emp_id AS 'Emp_ID'
, e1.emp_name AS 'Emp_Name'
, e2.emp_id AS 'Manager_ID'
, e2.emp_name AS 'Manager_Name'
FROM Employee e1 RIGHT JOIN Employee e2 ON e1.emp_id = e2.emp_id
Keep in mind sometimes you want e1.manager_id > e2.id
The advantage to knowing both scenarios is sometimes you have a ton of WHERE or JOIN conditions and you want to place your self join conditions in the other clause to keep your code readable.
No one addressed what happens when an Employee does not have a manager. Huh? They are not included in the result set. What if you want to include employees that do not have managers but you don't want incorrect combinations returned?
Try this puppy;
SELECT e1.emp_id AS 'Emp_ID'
, e1.emp_name AS 'Emp_Name'
, e2.emp_id AS 'Manager_ID'
, e2.emp_name AS 'Manager_Name'
FROM Employee e1 LEFT JOIN Employee e2
ON e1.emp_id = e2.emp_id
AND e1.emp_name = e2.emp_name
AND e1.every_other_matching_column = e2.every_other_matching_column

Self-join is useful when you have to evaluate the data of the table with itself. Which means it'll correlate the rows from the same table.
Syntax: SELECT * FROM TABLE t1, TABLE t2 WHERE t1.columnName = t2.columnName
For example, we want to find the names of the employees whose Initial Designation equals to current designation. We can solve this using self join in following way.
SELECT NAME FROM Employee e1, Employee e2 WHERE e1.intialDesignationId = e2.currentDesignationId

One use case is checking for duplicate records in a database.
SELECT A.Id FROM My_Bookings A, My_Bookings B
WHERE A.Name = B.Name
AND A.Date = B.Date
AND A.Id != B.Id

SELF JOIN:
Joining a table by itself is called as Self Join.
We can perform operations on a single table.
When we use self join we should create alias names on a table otherwise we cannot implement self join.
When we create alias name on a table internally system is preparing virtual table on each alias name of a table.
We can create any number of alias names on a table but each alias name should be different.
Basic Rules of self join:
CASE-I: Comparing a single column values by itself with in the table
CASE-II: Comparing two different columns values to each other with in the table.
Example:
SELECT * from TEST;
ENAME
RICHARD
JOHN
MATHEW
BENNY
LOC
HYDRABAD
MUMBAI
HYDRABAD
CHENNAI
SELECT T1. ENAME, T1. LOC FROM TEST.T1, TEST T2 WHERE T1.LOC=T2.LOC AND T2.ENAME='RICHARD';

It's the database equivalent of a linked list/tree, where a row contains a reference in some capacity to another row.

Here is the exaplanation of self join in layman terms. Self join is not a different type of join. If you have understood other types of joins (Inner, Outer, and Cross Joins), then self join should be straight forward. In INNER, OUTER and CROSS JOINS, you join 2 or more different tables. However, in self join you join the same table with itslef. Here, we don't have 2 different tables, but treat the same table as a different table using table aliases. If this is still not clear, I would recomend to watch the following youtube videos.
Self Join with an example