select statement with only rows which have set true in second table - mysql

Question

i have two tables
activity
id | user_id | time | activity_id
1 | 1 | | 3
2 | 1 | | 1
and preferences
user_id | running | cycling | driving
1 | TRUE | FALSE | FALSE
i need result set of
id | user_id | time |
2 | 1 | |
i only need rows from first table whose values are set true in preferences table.
e.g activity_id for running is 1 which is set true in preferences table, so it returns while others doesn't.

Answer 1

If you can edit the schema, it would be better like this:
activity
id | name
1 | running
2 | cycling
3 | driving
user_activity
id | user_id | time | activity_id
1 | 1 | | 3
2 | 1 | | 1
preferences
user_id | activity_id
1 | 1
A row in preferences indicates a TRUE value from your schema. No row indicates a FALSE.
Then your query would simply be:
SELECT ua.id, ua.user_id, ua.time
FROM user_activity ua
JOIN preferences p ON ua.user_id = p.user_id
AND ua.activity_id = p.activity_id
If you want to see the activity name in the results:
SELECT ua.id, ua.user_id, ua.time, activity.name
FROM user_activity ua
JOIN preferences p ON ua.user_id = p.user_id
AND ua.activity_id = p.activity_id
JOIN activity ON ua.activity_id = activity.id

Answer 2

What I would probably do is join the tables on a common column, looks like user_id is a common column in this case, which gives access to the columns in both tables to query against in the where clause of the query.
Which type of join depends on what information you want from preferences
Handy Visual Guide for joins
So you could query
SELECT * FROM activity LEFT JOIN preferences ON activity.user_id = preferences.user_id WHERE preferences.columnIWantToBeTrue = true
I'm using left join since you mentioned you want the values from the first table based on the second table.

Answer 3

Mike B has the right answer. The relational model relates rows together by common values.
You've got a table named activity with an id column which looks like the primary key. The column name activity_id would typically be the name of a column in another table that is a foreign key to the activity table, referencing activity.id.
It looks like you've used the activity_id column in the activity table as a reference to either "running", "cycling" or "driving".
It's possible to match activity.activity_id = 1 with "running", but this is a bizarre design.
Here's an example query:
SELECT a.id
, a.user_id
, a.time
FROM activity a
JOIN preferences p
ON p.user_id = a.user_id
AND ( ( p.running = 'TRUE' AND a.activity_id = 1 )
OR ( p.cycling = 'TRUE' AND a.activity_id = 2 )
OR ( p.driving = 'TRUE' AND a.activity_id = 3 )
)
But, again, this is a bizarre design.
As a start, each table in your database should have rows that represent either an entity (a person, place, thing, concept or event that can be uniquely identified, is important, and we need to store information about), or a relationship between the entities.
From the limited information we have about your use case, the entities appear to be "user", an "activity_type" (running, cycling, driving), an "activity" (an amount of time, for a user and an activity_type) and some user "preference" about which activity_types the user prefers.
See the answer from Mark B for a possible schema design.