How would you reference table1 columns to 2 columns in table 2
I created a table 'State' with 50 exact rows
trying to relate (weddingState,contactState) in 'Wedding' table
This is the statement that I created but it only joins the top WeddingState correctly - seems not to care about the INNER Join below it...
SELECT *
FROM weddings
INNER JOIN states as s1 ON weddings.WeddingState = s1.StateId //state of marriage
INNER JOIN states as s2 ON weddings.ContactState = s2.StateId //contact state of bride
WHERE weddings.weddingid="094829292"
I'd guess that you're retrieving these in PHP or something, and you're fetching the rows in a hash-array, keyed by the field name. Of course there can only be one element in a hash with a given key. So you need to use column aliases to make sure columns with the same name are given a distinct alias.
SELECT w.*, s1.StateID AS wstate, s2.StateId AS cstate
FROM weddings AS w
INNER JOIN states AS s1 ON w.WeddingState = s1.StateId //state of marriage
INNER JOIN states AS s2 ON w.ContactState = s2.StateId //contact state of bride
WHERE w.weddingid="094829292";
Now your hash-array will have keys "wstate" and "cstate". Without aliasing these columns, one will always overwrite the other.
And what are you getting for your result that leads you to your conclusion?
It's going to be confusing for starters because the field names in the two joins, plus some of the field names in the primary table, are identical. It's a really good idea to explicitly choose your output columns and give them meaningful aliases.
How about:
SELECT s1.StateName, s2.StateName
FROM weddings
INNER JOIN states as s1 ON weddings.WeddingState = s1.StateId //state of marriage
INNER JOIN states as s2 ON weddings.ContactState = s2.StateId //contact state of bride
WHERE weddings.weddingid="094829292"
Thanks Bill, I added the StateName as well
SELECT w.*,
s1.StateId AS WeddingStateId,
s1.StateName AS WeddingStateName,
s2.StateId AS ContactStateId,
s2.StateName AS ContactStateName
FROM weddings AS w
INNER JOIN states AS s1 ON w.WeddingState = s1.StateId
INNER JOIN states AS s2 ON w.ContactState = s2.StateId
Related
Hopefully you are going to help me again :)
Well, the problem that I have is similar to the one which I've posted yesterday, but it is extended.
We are given three tables:
Pfleger
Station
Mitarbeiter
As states above, this problem is very similar to this problem.
What was the result? Well, I get back a table with the ID's and names of the workers who are living in Frankfurt.
Now I should additionally get back the ID'S and names of the workers who are living in Frankfurt AND working in the station called Onkologie.
How should I do this?
My code so far:
SELECT pfleger.PNR, Name
from mitarbeiter, ...
JOIN pfleger on (mitarbeiter.PNR=pfleger.PNR)
JOIN ...
where Ort='Frankfurt' and Name='Onkologie'
I don't know how to make 2nd JOIN.
You could try something like this
select m.PNR, m.Name
from Mitarbeiter m
inner join Station s on s.PNR = m.PNR
inner join Pfleger p on p.StationID = s.StationID
where
m.Ort = 'Frankfurt'
and p.Name = 'Onkologie'
Updated based on provided table names and column names for tables.
Notice: we eliminated the , notation after mitarbeiter, in your base query.
I don't see PNR in mitabeiter so I'm assuming stationID is how they join.
SELECT *
FROM pfleger P
INNER JOIN Station S
on S.StationId = P.StationID
INNER JOIN mitarbeiter M
on M.pnr = S.pnr
WHERE M.ORT='Frankfurt' and P.name = 'Onkologie'
Assumptions I made:
pfleger.stationID has a foreign key relationship to station.stationID
mitarbeiter.PNR has a foreign key relationship to station.PNR
We used inner joins here as we only want Mitarbeiter who exist in all 3 tables.
Otherwise we could use outer joins and return those who don't have records as well. (meaning it is unknown where they work it MIGHT be 'Onkologie' but we don't know as there is no record.)
I am using the 3 following tables:
First table
id
response
Second table
responseid
patientid
Third table
patientid
The relationship between first and second table is on id and responceid.
The relationship between third and second is on patientid.
Now I need to retrieve values from these tables like all values from first and third tables with the help of matching with patientid from second and 3rd table.
How can I do this?
Basically if all of the columns that defines their relationship are not nullable, then INNER JOIN will suffice. But if they are nullable and you still want to display all records from firstTB, you need to use LEFT JOIN instead of INNER JOIN.
SELECT a.*, b.*, c.*
FROM firstTB a
INNER JOIN secondTB b
ON a.ID = b.responceID
INNER JOIN thirdTB c
ON b.patientID = c.patientID
To further gain more knowledge about joins, kindly visit the link below:
Visual Representation of SQL Joins
You're probably looking for INNER JOIN or JOIN in general:
SELECT
response.id,
response.responce,
patient.patientid
FROM
`response_table` as `response`
INNER JOIN
`relation_table` as `relation`
ON
relation.responceid = response.id
INNER JOIN
`patient_table` as `patient`
ON
relation.patientid = patient.patientid
try
SELECT first.*
, third.*
FROM first
INNER JOIN second ON ( second.responseid = first.id )
INNER JOIN third ON ( third.patientid = second.patientid )
;
honestly, and no insult intended, if you have difficulties in coming up with queries like this one on your own, consider some training on db basics and db development, the sooner the better (just hoping i haven't blundered myself ... ;-)).
I have 2 tables:
table_name: user_tracking
With columns: id, bill_no, container_type, origin_id, destination_id
table_name: sea_ports
With columns: id, bill_no, port_name
I want to write a single query to get the origin port_name and the destination port_name.
my query is :
select a.container_type,
b.port_name as origin_port
from user_tracking a
left join sea_ports b
on a.bill_no = b.bill_no
where a.bill_number = '$bill_no'
How do I join the two columns origin_id and destination_id on the same field id from the table sea_ports to get two different outputs?
You need to join the table sea_ports twice so you can get the port_name for each origin and destination. And One more thing, I guess, you need need to use INNER JOIN rather than LEFT JOIN because there will always be destination and origin right? :D
SELECT a.ID,
a.bill_no,
a.container_type,
b.port_name AS Origin_name,
c.port_name AS Destination_name
FROM user_tracking a
INNER JOIN sea_ports b
ON a.origin_id = b.id
INNER JOIN sea_ports c
ON a.destination_id = c.id
WHERE a.bill_number = '$bill_no'
As a sidenote, the query is vulnerable with SQL Injection if the value(s) came from the outside. Please take a look at the article below to learn how to prevent from it. By using PreparedStatements you can get rid of using single quotes around values.
How to prevent SQL injection in PHP?
To filter a table output of selected entries from a single table i would need something like a multiple JOIN request through several tables.
I want to filter a table of people by a special column in the table. Lets say this column is "tasks." Now tasks is also another table with the column "people" and the values between those two tables are connected with an existant "join" table in the database, which is matching several IDs of one table to each ID of the other table.
Now if this would be simple as that i could just filter with an INNER JOIN and a special condition. The problem is, that the entries of the table "tasks" are connected to another table over a "join" table in the database. To simplify things lets say it is "settings". So each "task" consists of several "settings" which are connected via a join table in their IDs.
So what is the input?
I got an array of IDs, which are representing the settings-ids i do not want to be shown.
What should be the output?
As already said i want a filtered output of "people" while the filter is "settings."
I want the sql request to return each entry of the table "people" with only joined tasks that are not joining any of the "setting-ids" from the array.
I hope you can help me with that.
Thanks in advance!
Example
Settings-Table:
1. Is in progress
2. Is important
3. Has unsolved issues
Tasks-Table: (settings.tasks is the join table between many tasks to many settings)
1. Task from 01.01.2012 - JOINS Settings in 1 and 3 (In progress + unsolved issues)
2. Task from 02.01.2012 - JOINS Settings in 2 (Is important)
3. Task from 03.01.2012 - JOINS Settings in 1 and 2 (...)
People-Table: (people.tasks is the join table between many people to many tasks)
1. Guy - JOINS Tasks in 1, 2, 3 (Has been assigned to all 3 tasks)
2. Dude - JOINS Tasks in 1 (Has been assigned to the Task from 01.01.2012)
3. Girl - JOINS Tasks in 2, 3 (...)
Now there is an array passed to a sql query
[2,3] should return noone because every person is assigned in a task that was either important or had unsolved issues!
[3] would return me only the person "Girl" because it is the only one that is assigned to tasks (2 and 3) that had no unsolved issues.
I hope it is clear now. :)
SELECT DISTINCT PEOPLE.*
FROM PEOPLE INNER JOIN PEOPLE_TASKS ON PEOPLE.PERSON_ID = PEOPLE_TASKS.PERSON_ID
WHERE TASK_ID NOT IN (SELECT DISTINCT TASK_ID
FROM TASK_SETTINGS
WHERE SETTING_ID = <Id you don't want>)
EDIT (for supplying multiple setting ids you don't want)
SELECT DISTINCT PEOPLE.*
FROM PEOPLE INNER JOIN PEOPLE_TASKS ON PEOPLE.PERSON_ID = PEOPLE_TASKS.PERSON_ID
WHERE TASK_ID NOT IN (SELECT DISTINCT TASK_ID
FROM TASK_SETTINGS
WHERE SETTING_ID IN (<Id you don't want>))
First you have to join table people and table tasks with the join table, let's call it people_tasks.
select distinct p.* from people p
inner join people_tasks pt on p.people_id = pt.people_id
inner join tasks on t.tasks_id = pt.tasks_id
Then you have to join table tasks and table settings with the join table, let's call it tasks_settings. You have to join them in the current select.
select distinct p.* from people p
inner join people_tasks pt on p.people_id = pt.people_id
inner join tasks on t.tasks_id = pt.tasks_id
inner join tasks_settings ts on t.tasks_id = ts.tasks_id
inner join settings s on s.settings_id = ts.settings_id
and now you have all people connected with its tasks and its settings. Finally you need the restriction. With the people with the settings selected, you choose the others like this:
select distinct p.people_id from people p
inner join people_tasks pt on p.people_id = pt.people_id
where p.people_id not in (
select distinct p2.people_id from people p2
inner join people_tasks pt2 on p2.people_id = pt2.people_id
inner join tasks t2 on t2.tasks_id = pt2.tasks_id
inner join tasks_settings ts2 on t2.tasks_id = ts2.tasks_id
inner join settings s2 on s2.settings_id = ts2.settings_id
where s2.settings_id in (list of ids)
)
I don't understand the concept of a left outer join, a right outer join, or indeed why we need to use a join at all! The question I am struggling with and the table I am working from is here: Link
Question 3(b)
Construct a command in SQL to solve the following query, explaining why it had to employ the
(outer) join method. [5 Marks]
“Find the name of each staff member and his/her dependent spouse, if any”
Question 3(c) -
Construct a command in SQL to solve the following query, using (i) the join method, and (ii) the
subquery method. [10 Marks]
“Find the identity name of each staff member who has worked more than 20 hours on the
Computerization Project”
Can anyone please explain this to me simply?
Joins are used to combine two related tables together.
In your example, you can combine the Employee table and the Department table, like so:
SELECT FNAME, LNAME, DNAME
FROM
EMPLOYEE INNER JOIN DEPARTMENT ON EMPLOYEE.DNO=DEPARTMENT.DNUMBER
This would result in a recordset like:
FNAME LNAME DNAME
----- ----- -----
John Smith Research
John Doe Administration
I used an INNER JOIN above. INNER JOINs combine two tables so that only records with matches in both tables are displayed, and they are joined in this case, on the department number (field DNO in Employee, DNUMBER in Department table).
LEFT JOINs allow you to combine two tables when you have records in the first table but might not have records in the second table. For example, let's say you want a list of all the employees, plus any dependents:
SELECT EMPLOYEE.FNAME as employee_first, EMPLOYEE.LNAME as employee_last, DEPENDENT.FNAME as dependent_last, DEPENDENT.LNAME as dependent_last
FROM
EMPLOYEE INNER JOIN DEPENDENT ON EMPLOYEE.SSN=DEPENDENT.ESSN
The problem here is that if an employee doesn't have a dependent, then their record won't show up at all -- because there's no matching record in the DEPENDENT table.
So, you use a left join which keeps all the data on the "left" (i.e. the first table) and pulls in any matching data on the "right" (the second table):
SELECT EMPLOYEE.FNAME as employee_first, EMPLOYEE.LNAME as employee_last, DEPENDENT.FNAME as dependent_first, DEPENDENT.LNAME as dependent_last
FROM
EMPLOYEE LEFT JOIN DEPENDENT ON EMPLOYEE.SSN=DEPENDENT.ESSN
Now we get all of the employee records. If there is no matching dependent(s) for a given employee, the dependent_first and dependent_last fields will be null.
example (not using your example tables :-)
I have a car rental company.
Table car
id: integer primary key autoincrement
licence_plate: varchar
purchase_date: date
Table customer
id: integer primary key autoincrement
name: varchar
Table rental
id: integer primary key autoincrement
car_id: integer
bike_id: integer
customer_id: integer
rental_date: date
Simple right? I have 10 records for cars because I have 10 cars.
I've been running this business for 10 years, so I've got 1000 customers.
And I rent the cars about 20x per year per cars = 10 years x 10 cars x 20 = 2000 rentals.
If I store everything in one big table I've got 10x1000x2000 = 20 million records.
If I store it in 3 tables I've got 10+1000+2000 = 3010 records.
That's 3 orders of magnitude, so that's why I use 3 tables.
But because I use 3 tables (to save space and time) I have to use joins in order to get the data out again
(at least if I want names and licence plates instead of numbers).
Using inner joins
All rentals for customer 345?
SELECT * FROM customer
INNER JOIN rental on (rental.customer_id = customer.id)
INNER JOIN car on (car.id = rental.car_id)
WHERE customer.id = 345.
That's an INNER JOIN, because we only want to know about cars linked to rentals linked to customers that actually happened.
Notice that we also have a bike_id, linking to the bike table, which is pretty similar to the car table but different.
How would we get all bike + car rentals for customer 345.
We can try and do this
SELECT * FROM customer
INNER JOIN rental on (rental.customer_id = customer.id)
INNER JOIN car on (car.id = rental.car_id)
INNER JOIN bike on (bike.id = rental.bike_id)
WHERE customer.id = 345.
But that will give an empty set!!
This is because a rental can either be a bike_rental OR a car_rental, but not both at the same time.
And the non-working inner join query will only give results for all rentals where we rent out both a bike and a car in the same transaction.
We are trying to get and boolean OR relationship using a boolean AND join.
Using outer joins
In order to solve this we need an outer join.
Let's solve it with left join
SELECT * FROM customer
INNER JOIN rental on (rental.customer_id = customer.id) <<-- link always
LEFT JOIN car on (car.id = rental.car_id) <<-- link half of the time
LEFT JOIN bike on (bike.id = rental.bike_id) <<-- link (other) 0.5 of the time.
WHERE customer.id = 345.
Look at it this way. An inner join is an AND and a left join is a OR as in the following pseudocode:
if a=1 AND a=2 then {this is always false, no result}
if a=1 OR a=2 then {this might be true or not}
If you create the tables and run the query you can see the result.
on terminology
A left join is the same as a left outer join.
A join with no extra prefixes is an inner join
There's also a full outer join. In 25 years of programming I've never used that.
Why Left join
Well there's two tables involved. In the example we linked
customer to rental with an inner join, in an inner join both tables must link so there is no difference between the left:customer table and the right:rental table.
The next link was a left join between left:rental and right:car. On the left side all rows must link and the right side they don't have to. This is why it's a left join
You use outer joins when you need all of the results from one of the join tables, whether there is a matching row in the other table or not.
I think Question 3(b) is confusing because its entire premise wrong: you don't have to use an outer join to "solve the query" e.g. consider this (following the style of syntax in the exam paper is probably wise):
SELECT FNAME, LNAME, DEPENDENT_NAME
FROM EMPLOYEE, DEPENDENT
WHERE SSN = ESSN
AND RELATIONSHIP = 'SPOUSE'
UNION
SELECT FNAME, LNAME, NULL
FROM EMPLOYEE
EXCEPT
SELECT FNAME, LNAME, DEPENDENT_NAME
FROM EMPLOYEE, DEPENDENT
WHERE SSN = ESSN
AND RELATIONSHIP = 'SPOUSE'
In general:
JOIN joints two tables together.
Use INNER JOIN when you wanna "look up", like look up detailed information of any specific column.
Use OUTER JOIN when you wanna "demonstrate", like list all the info of the 2 tables.