SQL - Column in field list is ambiguous - mysql

Question

I have two tables BOOKINGS and WORKER. Basically there is table for a worker and a table to keep track of what the worker has to do in a time frame aka booking. I’m trying to check if there is an available worker for a job, so I query the booking to check if requested time has available workers between the start end date. However, I get stuck on the next part. Which is returning the list of workers that do have that time available. I read that I could join the table passed on a shared column, so I tried doing an inner join with the WORKER_NAME column, but when I try to do this I get a ambiguous error. This leads me to believe I misunderstood the concept. Does anyone understand what I;m trying to do and knows how to do it, or knows why I have the error below. Thanks guys !!!!
CREATE TABLE WORKER (
ID INT NOT NULL AUTO_INCREMENT,
WORKER_NAME varchar(80) NOT NULL,
WORKER_CODE INT,
WORKER_WAGE INT,
PRIMARY KEY (ID)
)
CREATE TABLE BOOKING (
ID INT NOT NULL AUTO_INCREMENT,
WORKER_NAME varchar(80) NOT NULL,
START DATE NOT NULL,
END DATE NOT NULL,
PRIMARY KEY (ID)
)
query
SELECT *
FROM WORKERS
INNER JOIN BOOKING
ON WORKER_NAME = WORKER_NAME
WHERE (START NOT BETWEEN '2010-10-01' AND '2010-10-10')
ORDER BY ID
#1052 - Column 'WORKER_NAME' in on clause is ambiguous

Answer 1

In your query, the column "worker_name" exists in two tables; in this case, you must reference the tablename as part of the column identifer.
SELECT *
FROM WORKERS
INNER JOIN BOOKING
ON workers.WORKER_NAME = booking.WORKER_NAME
WHERE (START NOT BETWEEN '2010-10-01' AND '2010-10-10')
ORDER BY ID

Answer 2

In your query, the column WORKER_NAME and ID columns exists in both tables, where WORKER_NAME retains the same meaning and ID is re-purposed; in this case, you must either specify you are using WORKER_NAME as the join search condition or 'project away' (rename or omit) the duplicate ID problem.
Because the ID columns are AUTO_INCREMENT, I assume (hope!) they have no business meaning. Therefore, they could both be omitted, allowing a natural join that will cause duplicate columns to be 'projected away'. This is one of those situations where one wishes SQL had a WORKER ( ALL BUT ( ID ) ) type syntax; instead, one is required to do it longhand. It might be easier in the long run to to opt for a consistent naming convention and rename the columns to WORKER_ID and BOOKING_ID respectively.
You would also need to identify a business key to order on e.g. ( START, WORKER_NAME ):
SELECT *
FROM
( SELECT WORKER_NAME, WORKER_CODE, WORKER_WAGE FROM WORKER ) AS W
NATURAL JOIN
( SELECT WORKER_NAME, START, END FROM BOOKING ) AS B
WHERE ( START NOT BETWEEN '2010-10-01' AND '2010-10-10' )
ORDER BY START, WORKER_NAME;
This is good, but its returning the start and end times as well. I'm just wanting the WOKER ROWS. I cant take the start and end out, because then sql doesn’t recognize the where clause.
Two approaches spring to mind: push the where clause to the subquery:
SELECT *
FROM
( SELECT WORKER_NAME, WORKER_CODE, WORKER_WAGE FROM WORKER ) AS W
NATURAL JOIN
( SELECT WORKER_NAME, START, END
FROM BOOKING
WHERE START NOT BETWEEN '2010-10-01' AND '2010-10-10' ) AS B
ORDER BY START, WORKER_NAME;
Alternatively, replace SELECT * with a list of columns you want to SELECT:
SELECT WORKER_NAME, WORKER_CODE, WORKER_WAGE
FROM
( SELECT WORKER_NAME, WORKER_CODE, WORKER_WAGE FROM WORKER ) AS W
NATURAL JOIN
( SELECT WORKER_NAME, START, END FROM BOOKING ) AS B
WHERE START NOT BETWEEN '2010-10-01' AND '2010-10-10'
ORDER BY START, WORKER_NAME;

Answer 3

This error comes after you attempt to call a field which exists in both tables, therefore you should make a reference. For instance in example below I first say cod.coordinator so that DBMS know which coordinator I want
SELECT project__number, surname, firstname,cod.coordinator FROMcoordinatorsAS co JOIN hub_applicants AS ap ON co.project__number = ap.project_id JOIN coordinator_duties AS cod ON co.coordinator = cod.email