两个表的内部连接

Question

As im trying to do an inner join, im getting stuck by an error : #1066 - Not unique table/alias: 'afspraken'

Here is what i am trying:

SELECT accounts.werknemer_id, accounts.voornaam, accounts.achternaam, klanten.klant_id,  klanten.voornaam, klanten.achternaam 
FROM accounts, klanten 
JOIN afspraken ON afspraken.werknemer_id = afspraken.werknemer_id JOIN afspraken ON afspraken.klant_id = afspraken.klant_id

i am wondering what is going wrong, i have been on google, but neither their i could find a solution.

Thanks in forehand

Answer 1

When you use an inner join of two tables, the condition of inner join must be an identical field existing in both. Also, the fields selected must be in one table or another, you cannot just place a condition to inner join based on the same field of the same table. the equal afspraken.werknemer_id = afspraken.werknemer_id and the equal afspraken.klant_id = afspraken.klant_id are causing the error.

I believe you want to join both tables accounts and klanten with no common field through the table afspraken with werknemer_id and klant_id fields, so it would be like this:

SELECT accounts.werknemer_id,
accounts.voornaam,
accounts.achternaam,
klanten.klant_id,
klanten.voornaam,
klanten.achternaam 
FROM accounts 
INNER JOIN afspraken ON accounts.werknemer_id = afspraken.werknemer_id 
INNER JOIN klanten ON afspraken.klant_id = klanten.klant_id

Answer 2

There are several parts of this query that are worrysome:

1. Comma join syntax vs inner join syntax (pick 1, you can't do both)
2. No table alias when joining two tables with the same name. Compiler can't figure out what to do.
3. Relationships between tables are not identifiable, thus the data returned will not likely produce what you're trying to do
4. afspraken is added to the query, but not in the select what purpose does this serve?
5. In plain English, describe what it is you need. (show sample data and desired results)

.

SELECT accounts.werknemer_id, accounts.voornaam, accounts.achternaam, klanten.klant_id,  klanten.voornaam, klanten.achternaam 
FROM accounts, klanten   <-- This line is the problem
JOIN afspraken ON afspraken.werknemer_id = afspraken.werknemer_id <--or these two are (1)
JOIN afspraken ON afspraken.klant_id = afspraken.klant_id <--(2)

to solve we would need to know how accounts relate to klanten and why you need two joins to afspraken when you could use an and clause on the join syntax such as...

INNER JOIN afspraken 
  ON afspraken.werknemer_id = afspraken.werknemer_id <--or these two are (1)
 AND afspraken.klant_id = afspraken.klant_id <--(2)

Furthermore, you don't alias afspraken. which also would cause problems as the system wouldn't know what join to link to.

So either...

SELECT A.werknemer_id, A.voornaam, A.achternaam, K.klant_id,  K.voornaam, K.achternaam 
FROM accounts A, klanten K, afspraken AF, afspraken AF2
WHERE AF.werknemer_id = AF2.werknemer_id 
 AND  AF.klant_id = AF2.klant_id 

Though again how do accounts relate to klaten OR

SELECT A.werknemer_id, A.voornaam, A.achternaam, K.klant_id,  K.voornaam, K.achternaam 
FROM accounts A
CROSS JOIN klanten K
CROSS JOIN afspraken AF, 
INNER JOIN afspraken AF2
  ON AF.werknemer_id = AF2.werknemer_id 
 AND AF.klant_id = AF2.klant_id 

but I likely have the relationships messed up as I don't understand how each table relates to each other table in your example.