SQL: Вложенный запрос выбора?

Question

Кто-нибудь знает, как я могу объединить их в одном утверждении? Это lesson_id (s) для всех сеансов с конкретного блока, что я после ...

$query = " 
SELECT session_id FROM cdu_blocks_sessions 
JOIN cdu_sessions ON cdu_sessions.id = cdu_blocks_sessions.session_id 
WHERE block_id = $block_id 
AND enabled = 1"; 

$session_ids = cdu_get_query_results($db, $query, __LINE__); 

$lesson_ids = array(); 

foreach ($session_ids as $session_id) { 
    $query = " 
    SELECT lesson_id FROM cdu_sessions_lessons 
    JOIN cdu_lessons ON cdu_lessons.id = cdu_sessions_lessons.lesson_id 
    WHERE session_id = " . $session_id['session_id'] . " 
    AND enabled = 1"; 

    $lesson_ids = array_merge($lesson_ids, cdu_get_query_results($db, $query, __LINE__)); 
} 

cdu_sessions:

------------------------------ 
|id |name   |enabled | 
------------------------------ 
|1 |session 1  |1  | 
|2 |session 2  |1  | 
|3 |session 3  |1  | 
|4 |session 4  |0  | 
|5 |session 5  |1  | 
|6 |session 6  |0  | 
|7 |session 7  |1  | 
|8 |session 8  |1  | 
------------------------------ 

cdu_blocks_sessions:

--------------------------- 
|id |block_id |session_id | 
--------------------------- 
|1 |1  |1   | 
|1 |1  |2   | 
|1 |1  |3   | 
|1 |2  |4   | 
|1 |2  |5   | 
|1 |2  |6   | 
--------------------------- 

cdu_lessons:

----------------------------- 
|id |name   |enabled | 
----------------------------- 
|1 |lesson 1  |1  | 
|2 |lesson 2  |1  | 
|3 |lesson 3  |1  | 
|4 |lesson 4  |1  | 
|5 |lesson 5  |0  | 
----------------------------- 

cdu_sessions_lessons:

---------------------------- 
|id |session_id |lesson_id | 
---------------------------- 
|1 |1   |1   | 
|1 |1   |2   | 
|1 |1   |3   | 
|1 |2   |4   | 
|1 |2   |5   | 
---------------------------- 

Answer 1

Попробуйте этот запрос для SQL Server

SELECT * 
FROM (
SELECT session_id FROM cdu_blocks_sessions 
JOIN cdu_sessions ON cdu_sessions.id = cdu_blocks_sessions.session_id 
WHERE block_id = $block_id 
AND enabled = 1 
) S 
CROSS APPLY (
SELECT lesson_id FROM cdu_sessions_lessons 
    JOIN cdu_lessons ON cdu_lessons.id = cdu_sessions_lessons.lesson_id 
    WHERE session_id = S.session_id 
    AND enabled = 1 
) L 

Больше на кресте применять

SQL Server CROSS APPLY and OUTER APPLY

Другой способ

SELECT L.* 
    FROM (
    SELECT session_id FROM cdu_blocks_sessions 
    JOIN cdu_sessions ON cdu_sessions.id = cdu_blocks_sessions.session_id 
    WHERE block_id = $block_id 
    AND enabled = 1 
    ) S 
    INNER JOIN (
     SELECT lesson_id, session_id FROM cdu_sessions_lessons 
     JOIN cdu_lessons ON cdu_lessons.id = cdu_sessions_lessons.lesson_id 
     WHERE enabled = 1 
    ) L S.session_id = L.session_id