MySQL错误1054未知列

Question

I have 2 tables.

1. t_problem
2. t_riesenia

In t_problem I have primary key id .
In t_riesenia I have foreign key ProblemID . At least I think I have.
I want to select all fields from table t_problem where there is a solution in t_riesenia (there is a ProblemID in t_riesenia equal to id in t_problem )

When trying to execute SQL statement :

SELECT * FROM `t_problem` where `t_problem`.`id`=`t_riesenia`.`ProblemID`  

I get an error :

Error Code: 1054. Unknown column 't_riesenia.ProblemID' in 'where clause'

Why? See screenshots from MySQL workbench below.

t_problem

t_riesenia

t_riesenia Foreign key

Answer 1

You should be using an explicit join :

SELECT p.*
FROM t_problem p JOIN
     t_riesenia r
      ON p.id = r.ProblemID;  

It is also possible that you intend an IN :

SELECT p.*
FROM t_problem p
WHERE p.id IN (SELECT r.ProblemID FROM t_riesenia r);

The difference between the two is how duplicates are handled in t_riesenia . The first returns duplicate rows. The second does not.

Answer 2

That column may exist somewhere in your database , but it doesn't exist in your query . You query specifies only one table:

SELECT * FROM `t_problem` where `t_problem`.`id`=`t_riesenia`.`ProblemID`

Note the FROM clause has only a single table. You can see it a little more clearly if you structure it explicitly:

SELECT
    *
FROM
    `t_problem`
WHERE
    `t_problem`.`id`=`t_riesenia`.`ProblemID`

The only known table in this query is t_problem . In order to add more tables, they need to be added to the FROM clause. Something like this, for example:

SELECT
    `t_problem`.*
FROM
    `t_problem`
    INNER JOIN `t_riesenia` ON `t_problem`.`id` = `t_riesenia`.`ProblemID`

(Note that in this case the WHERE clause isn't really needed, since that comparison is going to be true for every joined record.)