如果连接表中没有匹配项，如何仍然从连接查询中的表中获取数据

Question

I have a database which contains two tables,

snm_content and snm_fields_values

snm_content has numerous fields, only id is relevant here, and snm_fields_values has three fields, item_id , field_id and value

the field id of snm_content is the same as the field item_id of snm_fields_values .

I want to get all data from snm_content and join snm_fields_values but also retrieve data from snm_content if nothing can be joined. So if there is no matching item_id in snm_fields_values then still get the data from snm_content .

The problem is with the following query I only get results from snm_content if there is a matching item_id in snm_fields_values

SELECT cnt.*,
MAX(case when f.field_id = 4 then f.value end) as service_icon
FROM snm_fields_values f
JOIN snm_content cnt
ON cnt.id = f.item_id
WHERE cnt.catid = 9
AND cnt.state = 1
ORDER BY cnt.ordering

If no item_id exists for an id in snm_content nothing is retrieved from snm_content . How can I fix that? I've tried switching the order of retrieving data but this had the same result.

Answer 1

Use a left join :

SELECT cnt.*,
       MAX(case when f.field_id = 4 then f.value end) as service_icon
FROM snm_content cnt LEFT JOIN
     snm_fields_values f
     ON cnt.id = f.item_id
WHERE cnt.catid = 9 AND cnt.state = 1
GROUP BY cnt.id
ORDER BY cnt.ordering;

Note that this adds GROUP BY so you get one row per group in the content table.

You can also use a correlated subquery:

SELECT cnt.*,
       (SELECT MAX(case when f.field_id = 4 then f.value end)
        FROM snm_fields_values f
        WHERE cnt.id = f.item_id
       ) as service_icon
FROM snm_content cnt 
WHERE cnt.catid = 9 AND cnt.state = 1
GROUP BY cnt.id
ORDER BY cnt.ordering;