every derived table must have it is own alias join mysql [duplicate] - mysql

Question

This question already has answers here:
What is the error "Every derived table must have its own alias" in MySQL?
(4 answers)
Closed 3 months ago.
on mysql I see this error but can fix it. Anyone can help please?
select * from
(
(select a.* from sessions as a)
join
(
select b.customer_id, min(b.timestamp),
b.marketing_source as first_touch_source,
b.marketing_medium as first_touch_medium
from sessions as b
group by b.customer_id
) on a.customer_id = b=customer_id
) as T

Answer 1

I believe your query should read
select *
from (
select a.*
from sessions as a
join
(select b.customer_id, min(b.timestamp), b.marketing_source as first_touch_source, b.marketing_medium as first_touch_medium
from sessions as b
group by b.customer_id
) c USING (customer_id) # or c.customer_id = a.customer_id
) as T

Answer 2

If you are just trying to bring in the minimum date column you could try two approaches, second one will work if your version supports windows function
subqueries are always a mess so I would suggest cte's if supported
with main as (
select
b.customer_id,
b.marketing_source as first_touch_source,
b.marketing_medium as first_touch_medium,
min(b.timestamp) as min_time,
from sessions as b
group by b.customer_id
)
select
s.*,
main.min_time
from sessions as s
inner join main
on s.customer_id = main.customer_id
select
*,
min(timestamp)
over(partition by customer_id,marketing_medium, marketing_source) as min_date_per_cust_med_source
from sessions

Answer 3

Some DBMS require you to name all derived tables. Your query (I removed the unnessesary derived table T):
select *
from (select a.* from sessions as a)
join (select b.customer_id, min(b.timestamp)
, b.marketing_source as first_touch_source
, b.marketing_medium as first_touch_medium
from sessions as b
group by b.customer_id
)
on a.customer_id = b=customer_id
can be changed to:
select *
from (select a.* from sessions as a) AS c
join (select b.customer_id, min(b.timestamp)
, b.marketing_source as first_touch_source
, b.marketing_medium as first_touch_medium
from sessions as b
group by b.customer_id
) AS d
on c.customer_id = d.customer_id
To avoid confusion, you should choose another alias at the outer level, despite that the inner alias is not visible there.
Side note: The derived table d may or may not be valid SQL. It is not allowed in SQL92, but it is allowed in SQL99 if marketing_* is functionally dependent of customer_id.
You can further simplify it as:
select *
from sessions AS c
join (select b.customer_id, min(b.timestamp) as ts
, b.marketing_source as first_touch_source
, b.marketing_medium as first_touch_medium
from sessions as b
group by b.customer_id
) AS d
on c.customer_id = d.customer_id
I assume you meant to also join with c.timestamp = d.ts. If that is the case and you are on a recent version of MySQL (8+) you can use a window function instead of a self join
select customer_id, ...
from (
select b.customer_id
, b.marketing_source
, b.marketing_medium
, row_number() over (partition by customer_id
order by b.timestamp) as rn
from sessions as b
) as T
where rn = 1