Question

我有以下sql处理1个需要帮助的小问题。即使连接的表之一为空，我也需要显示结果。我曾尝试使用左/右联接，但似乎没有什么不同，我需要使用什么来完成这项工作？

现在，查询将一直有效，除非表dx_code_ Patient中的记录不匹配，即使该表中没有匹配的记录，我也需要显示结果。

SELECT
 group_concat(distinct current_dx.dx_code_with, ': ', current_dx.description SEPARATOR ' - ') AS current_dxc, 
 group_concat(distinct pending_dx.dx_code_with, ': ', pending_dx.description SEPARATOR ' - ') AS pending_dxc,
 p.id, p.first_name, p.last_name
 FROM patients AS p
 INNER JOIN tmp_dx_code_patient AS tmp_dx
   ON tmp_dx.patient_id = p.id    
 INNER JOIN dx_code_patient AS cdx 
   ON cdx.patient_id = p.id
 INNER JOIN dx_codes AS current_dx
   ON current_dx.id = cdx.dx_code_id
 INNER JOIN dx_codes AS pending_dx
   ON pending_dx.id = tmp_dx.dx_code_id
 GROUP BY p.id
 ORDER BY tmp_dx.created_at asc

当前结果：

+----------------+--------------+----+------------+-----------+
| current_dxc    | pending_dxc  | id | first_name | last_name |
+----------------+--------------+----+------------+-----------+
| def: something | 123: message |  2 | Bob        | Smith     |
+----------------+--------------+----+------------+-----------+

预期

+----------------+---------------+----+------------+------------+
| current_dxc    | pending_dxc   | id | first_name | last_name  |
+----------------+---------------+----+------------+------------+
| null           | ghy: hi       |  1 |       Mike |      Jones |
+----------------+---------------+----+------------+------------+
| def: something | 123: message  |  2 |        Bob |      Smith |
+----------------+---------------+----+------------+------------+
| null           | 432: question |  3 |       John |        Doe |
+----------------+---------------+----+------------+------------+

Answer 1

JOIN不返回NULL结果。我的猜测是您的意思是没有行匹配。如果是这样，LEFT JOIN应该可以解决您的问题。在整个FROM子句中使用它：

FROM patients p LEFT JOIN
    tmp_dx_code_patient  tmp_dx
    ON tmp_dx.patient_id = p.id LEFT JOIN  
    dx_code_patient cdx 
    ON cdx.patient_id = p.id LEFT JOIN
    dx_codes current_dx
    ON current_dx.id = cdx.dx_code_id LEFT JOIN
    dx_codes pending_dx
    ON pending_dx.id = tmp_dx.dx_code_id

MySQL-即使join为null也检索结果

1 个答案: