从多个表中选择时，MySQL JOIN 会导致“错误 1054 (42S22)：未知列”

Question

Given the following schema:

CREATE TABLE Customers
(
    CustomerID INT,
    FirstName VARCHAR(32),
    LastName VARCHAR(32)
);

CREATE TABLE Programs
(
    ProgramID INT,
    ProgramName VARCHAR(32)
);

CREATE TABLE Registrations
(
    RegistrationID INT,
    ProgramID INT,
    CustomerID INT
    FirstName VARCHAR(32),
    LastName VARCHAR(32)
);

I can't think of any reason why this query should fail:

mysql> SELECT *
FROM Registrations R, Programs P
LEFT OUTER JOIN Customers C
ON R.CustomerID = C.CustomerID
WHERE R.ProgramID = P.ProgramID;

ERROR 1054 (42S22): Unknown column 'C.CustomerID' in 'on clause'

But running the query by selecting only the one table Registrations causes no errors:

mysql> SELECT *
FROM Registrations R
LEFT OUTER JOIN Customers C
ON R.CustomerID = C.CustomerID;

Empty set (0.00 sec)

What am I missing?

Before you suggest adding Customers to FROM, CustomerID is a "loose" association. For reasons not relevant to this problem, customer data sometimes needs to be decoupled from the registration data with a NULL value for CustomerID.

Answer 1

the problem is related to the use of a mix of implicit and explicit join
You can't use implict join based on list of comma separated table name and where clause and explicit join in the same query ..

SELECT *
FROM Registrations R
INNER JOIN  Programs P ON R.ProgramID = P.ProgramID
LEFT OUTER JOIN Customers C ON R.CustomerID = C.CustomerID