根据其他表的条件从表中选择数据

Question

I'm really unsure how best to go about writing this query. I have 3 tables and I need to run a query pulling data from one, based on conditions in the others.

Tables: surveys, survey_countries, survey_categories

surveys :
id | survey_id | network_id

survey_countries :
id | survey_id | network_id | country

survey_categories : id | survey_id | network_id | category

(The survey_id in survey_countries and survey_categories relates to the survey_id column in the surveys table as opposed to the id column).

I need to retrieve data from surveys for a specific country and a specific category. Then I need to be able to UNION for other categories, but I guess I can do that later. My attempt:

SELECT surveys.*
FROM survey_countries
LEFT JOIN surveys ON survey_countries.survey_id = surveys.survey_id ANd survey_countries.network_id = surveys.network_id
LEFT JOIN survey_categories ON survey_countries.survey_id = surveys.survey_id AND survey_categories.network_id = surveys.network_id
WHERE survey_countries.country = 'GB'
AND survey_categories.category = 'my_category'
GROUP BY surveys.id

Thanks!

EDIT: the following seems to work:

SELECT s.*, ca.category, co.country
FROM surveys s
LEFT JOIN survey_countries co ON s.survey_id = co.survey_id AND s.network_id = co.network_id
LEFT JOIN survey_categories ca ON s.survey_id = ca.survey_id AND s.network_id = ca.network_id
WHERE ca.category = 'uncategorised'
AND co.country = 'GB';

I'm just not sure it's the best way to do it since I need to grab surveys with multiple categories later on?

Answer 1

Without example data and an example output, helping you on this query is very difficult. However..... your 2nd join relates survey_countries with surveys. I think you mean to join it to the survey_categories. Also, you have a Group By, with no aggregate functions. Get rid of it until you have too much information an need to summarize columns

I am not saying this is correct, but it is slightly more 'correct' than yours

SELECT surveys.*
FROM survey_countries
LEFT JOIN surveys 
  ON survey_countries.survey_id = surveys.survey_id 
    AND survey_countries.network_id = surveys.network_id
LEFT JOIN survey_categories 
  ON survey_categories.survey_id = surveys.survey_id 
    AND survey_categories.network_id = surveys.network_id
WHERE survey_countries.country = 'GB'
  AND survey_categories.category = 'my_category'

(If you can, load up a schema and sample data in sqlfiddle. It will make this problem easier to solve.)