I have the below table T in MySQL, with columns A and B.
I want to query this table to get a value 'C', that has the count of the number of times the value in Column 'A' appears in Column 'B'. For example, 1 appears 2 times in Column B, so the first row of column C should be 2. I don't want to iterate over the table, so I want to get this using subqueries. The desired output is given below.
I tried using the below query
SELECT A, B, (SELECT COUNT(A) FROM T WHERE B = A) AS C
FROM T
But I got 0 for all rows in column C. Please help me identify the mistake.
Use a correlated subquery:
SELECT t1.A, t1.B,
(SELECT COUNT(*) FROM tablename t2 WHERE t2.B = t1.A) AS C
FROM tablename t1
Or:
SELECT t1.A, t1.B,
(SELECT SUM(t2.B = t1.A) FROM tablename t2) AS C
FROM tablename t1
Or with a self LEFT join and aggregation:
SELECT t1.A, t1.B, COUNT(t2.B) AS c
FROM tablename t1 LEFT JOIN tablename t2
ON t2.B = t1.A
GROUP BY t1.A, t1.B
Related
I have 2 tables, one is table1
and another is table 2
I want the result by a query, like
I have tried select id from table2 order by (select id from table1); but it is giving error.
You can join and sort. But you need a column that defines the ordering of the rows in table1. Let me assume that you have such column, and that is is called ordering_id.
select t2.*
from table2 t2
inner join table1 t1 on t1.id = t2.id
order by t1.ordering_id
You can even use a subquery in the order by clause:
select *
from table2 t2
order by (select t1.ordering_id from table1 t1 where t1.id = t2.id)
Join the two tables and then order the result.But for that you need to have some column for ordering and this does not seems to be the case. Syntax you are using for ordering will not work.
SELECT A.ID, B.NAME FROM TABLE1 A INNER JOIN TABLE2 B
ON(A.ID = B.ID) ORDER BY A.ID DESC
finally got the answer
select t2.*
from table2 t2
inner join table1 t1 on t1.id = t2.id;
How can I write a query to give the results of three tables such that there's only one result per "line"?
The tables are:
T1 (ID, name, IP)
T2 (ID, date_joined)
T3 (ID, address, date_modified)
The relations are:
T1-T2 1:1, T1-T3 1:M - there can be many address rows per ID in T3.
What I want is a listing of all users with the fields above, but IF they have an address, I only want to record ONE (bonus would be if it is the latest one based on T3.date_modified).
So I should end up with exactly the number of records in T1 (happens to be equal to T2 in this case) and no more.
I tried:
select t.ID, t.name, t.IP, tt.ID, tt.date_joined, ttt.ID, ttt.address
from T1 t JOIN T2 tt ON (t.ID = tt.ID) JOIN T3 ttt ON (t.ID = ttt.ID)
And every sensible combination of LEFT, RIGHT, INNER, etc joins I could think of! I keep getting multiple duplicate because of T3
This query should work:
select
t1.ID, t1.name, t1.IP, t2.date_joined, t3x.address
from t1
join t2 on t1.ID = t2.id
left join (
select t3.*
from t3
join (
select id, max(date_modified) max_date
from t3
group by id
) max_t3 on t3.id = max_t3.id and t3.date_modified = max_t3.max_date
) t3x on t1.ID = t3x.id
First you do the normal join between t1 and t2 and then you left join with a derived table (t3x) that is the set of t3 rows having the latest date.
So T2 is actually not relevant here. You just need a way to join from T1 to T3 in a way that gets you at most one T3 row per T1 row.
One way of doing this would be:
select
T1.*,
(select address from T3 where T3.ID=T1.ID order by date_modified desc limit 1)
from T1;
This won't likely be very efficient, being a correlated subquery, but you may not care depending on the size of your dataset.
It's also only good for getting one column from T3, so if you had Address, City, and State, you'd have to figure out something else.
You can use sub query with Top 1 so that u get only one result from T3
here is a sample sql
select * into #T1 from(
select 1 ID
union select 2
union select 3) A
select * into #T2 from(
select 1 ID
union select 2
union select 3) A
select * into #T3 from(
select 1 ID, 'ABC' Address, getDate() dateModified
union select 1, 'DEF', getDate()
union select 3, 'GHI', getDate()) A
select *, (select top 1 Address from #T3 T3 where T3.ID= T1.ID order by datemodified desc) from #T1 T1
inner join #T2 T2 on T1.ID = T2.ID
Bonus :- you can also add order by dateModified desc to get the latest address
Table column headers: n,t1,t2
entries :
1 A B
2 A C
3 B C
4 D E
5 B A
How do I count total number of rows each letter appears in t1 MINUS the number of rows they appear in t2 ? I need to do something like following 2 lines in 1 query :
select count(*) as val,t1 from table group by t1
select count(*) as val,t2 from table group by t2
Thanks,
Martin
Here is one way:
select t1, max(t1cnt) - max(t2cnt) as diff
from ((select t1, count(*) as t1cnt, 0 as t2cnt
from t
group by t1
) union all
(select t2, 0 as t1cnt, count(*) as t2cnt
from t
group by t2
)
) t
group by t1
Using the union all ensures that you get all possible values from both columns, even values that only appear in one column.
You can use the following query to get the result. This query first gets a list of all the distinct t1 and t2 values (this is the UNION query). Once you have the list of these values, then you can use a LEFT JOIN to the original queries that you posted:
select d.col, coalesce(totT1, 0) - coalesce(totT2, 0) Total
from
(
select t1 col
from entries
union
select t2 col
from entries
) d
left join
(
select count(*) totT1, t1
from entries
group by t1
) d1
on d.col = d1.t1
left join
(
select count(*) totT2, t2
from entries
group by t2
) d2
on d.col = d2.t2;
See SQL Fiddle with Demo
I have two columns A and B. I want to select the value of column A where B has its maximum value.
SELECT MAX(B) FROM table_name
just gives me the value of B.
I can do another query with the value of B to get A, but is there a shorter way with just one SQL query?
select A
from MyTable
where B = (SELECT MAX(B) FROM MyTable)
or
select t.A
from MyTable t
inner join (
SELECT MAX(B) as BMAX FROM MyTable
) tm on t.B = t.BMAX
SELECT A
FROM table_name
WHERE B = (SELECT MAX(B) FROM table_name)
Consider the following DB table:
c p
=========
1 'a'
1 'b'
2 'a'
2 'c'
Now, my goal is to retrieve a list of numbers c, for which holds that each number in this list has at least a record with p='a' AND p='b'.
In the example table above, that would be c=1.
Now my question is, how do I accomplish this using one MySQL query?
select t1.c
from MyTable t1
inner join MyTable t2 on t1.c = t2.c
where t1.p = 'a' and t2.p = 'b'
Update:
select c
from MyTable
where p in ('a', 'b', 'c', 'd')
group by c
having count(distinct p) = 4
There are different ways to attack the problem depending on the rules your data follows if any. Without knowing more about your problem, I would do:
SELECT t1.c FROM table t1 INNER JOIN table t2
ON t1.c = t2.c
WHERE t1.p = 'a' AND t2.p = 'b'