I have a table1 (a,b,c) and table2(a,b,c)
What's the difference between
select * from table1 T1 inner join table2 T2 on
T1.a=T2.a and T1.b = t2.b and T1.c = T2.c
and
select * from table1 T1 inner Join table2 T2 on T1.a = T2.a where
T1.b= T2.b and T1.c = T2.C
Is is the same ? and which one is better?
Thanks
With inner joins there are no difference. It is only when you start using left/right joins, that you will see differences.
For LEFT JOINS, if you had
select *
from table1 T1 LEFT join
table2 T2 on T1.a=T2.a
and T1.b = t2.b
and T1.c = T2.c
It would include all rows fromo table1 and only rows from table2 where fields a,b and c matched.
If you had
select *
from table1 T1 inner Join
table2 T2 on T1.a = T2.a
where T1.b= T2.b
and T1.c = T2.C
This would include rows from table1 and those from table2 where a is equal, and then filter on b and c.
SQL Fiddle DEMO
I always find this visual representation usefull.
SQL SERVER – Introduction to JOINs – Basic of JOINs
For your queries, this doesn't change anything.
And in term of performance, your RDBMS is able to understand that it's the same.
But considering you have more joins, this would change the readability of the query.
Example :
SELECT
*
FROM
table1 T1
INNER JOIN table2 T2
ON T1.a = T2.a
AND T1.b = T2.b
AND T1.c = T2.c
LEFT JOIN table3 T3
ON T3.x = T1.a
AND T3.status = 1
WHERE
T1.a > 100
You can understand faster which condition works with which table in INNER/LEFT JOIN
Related
My first query...
SELECT
t1.a, t1.b, t1.c,
t2.a, t2.b, t2.c
FROM t1
LEFT JOIN t2 ON t1.a = t2.a
WHERE t1.b = '000000'
AND LENGTH(t1.a) > '5'
AND t1.c <> 'Y';
My second query...
SELECT
t1.a,
t3.b as testMe
FROM t1
LEFT JOIN t3 ON t1.a = '0' + LEFT(t3.a, 5)
WHERE t1.a = '017941';
Both of these queries work fine by themselves, but I need them combined into one result set. Worth noting is that the where clause in the second query is there for testing purposes, but when I remove it the whole thing crashes. Not sure if that means I need something to filter by, or it's timing out? The database I'm using is Pervasive.
My failed query...
SELECT
t1.a, t1.b, t1.c,
t2.a, t2.b, t2.c,
t3.b as testMe
FROM t1
LEFT JOIN t2 ON t1.a = t2.a
LEFT JOIN t3 ON t1.a = '0' + LEFT(t3.a, 5)
WHERE t1.b = '000000'
AND LENGTH(t1.a) > '5'
AND t1.c <> 'Y';
I've shorted my code to better show what I have going on, but the actual code can be found here: http://codeshare.io/A2aB9
You can try to use those two sets as subselect statements and join them together. I pulled your join condition between the two queries out as a separate column in order to keep testMe in the result set.
SELECT *
FROM (SELECT t1.a t1_a
, t1.b t1_b
, t1.c t1_c
, t2.a t2_a
, t2.b t2_b
, t2.c t2_c
FROM t1
LEFT JOIN t2
ON t1.a = t2.a
WHERE t1.b = '000000'
AND LENGTH(t1.a) > '5'
AND t1.c <> 'Y') a
LEFT JOIN (SELECT t1.a t1_a
, t1.b t1_b
, t1.c t1_c
, t3.a t3_a
, t3.b t3_b
, '0' + LEFT(t3.c, 5) t3_c
, LEFT(t3.c, 5) AS testMe
FROM t1
LEFT JOIN t3
ON t1.a = '0' + LEFT(t3.c, 5)) b
ON a.t1_a = c.t3_c;
I got three tables:
t1.columns: a,c
t2.columns: a,b
t3.columns: b,c,d
Now what I want is to update t1.c with t3.d. But I can't just update t1 from t3 using t1.c = t3.c I also have to go though t3.b = t2.b and t1.a = t2.a.
I've tried something like this:
UPDATE table1 t1
SET t1.c = (select t3.d
from table2 t2, table3 t3
where t2.b = t3.b and t1.a = t2.a)
WHERE EXISTS ( SELECT 1 FROM table2 t2, table3 t3 WHERE t1.c = t3.c and t1.a = t2.a);
This code generates error-msg: ORA-01427: single-row subquery returns more than one row
If there is a one-to-many relationship between t1 and t2 or between t2 and t3 you will get many matches for each row in t1. If you know that all rows in t3 that belong to the same row in t1 have the same value in d, then you can use DISTINCT to remove (identical) duplicates.
UPDATE table1 t1
SET t1.c = (select DISTINCT t3.d
from table2 t2, table3 t3
where t2.b = t3.b and t1.a = t2.a)
WHERE EXISTS ( SELECT 1 FROM table2 t2, table3 t3 WHERE t1.c = t3.c and t1.a = t2.a);
You have a subquery that is returning more than one row. Use rownum to get just one row:
UPDATE table1 t1
SET t1.c = (select d
from (select t3.d
from table2 t2 join table3 t3
on t2.b = t3.b
where t1.a = t2.a
) t
where rownum = 1
)
WHERE EXISTS ( SELECT 1 FROM table2 t2, table3 t3 WHERE t1.c = t3.c and t1.a = t2.a);
Sorry for the confusion but I solved it:
UPDATE table t1
SET t1.c = (select t3.d from table3 t3, table2 t2
where t1.a = t2.a and t2.b = t3.b and t3.c = t1.c)
WHERE EXISTS ( SELECT 1 FROM table1 t1, table2 t2 WHERE t1.a = t2.a and t2.b = t3.b and t3.c = t1.c)
UPDATE table1 t1
SET t1.c = (select MAX(t3.d)
from table2 t2, table3 t3
where t2.b = t3.b and t1.a = t2.a)
WHERE EXISTS ( SELECT 1 FROM table2 t2, table3 t3 WHERE t1.c = t3.c and t1.a = t2.a);
I have 2 tables as follows T1 and T2.
T1 has one field as A and T2 has one field B.
Now i want to do following: for each value of T1.A I want to join with T2.B
Something like :
select * from T1 ,(select * from where T2 where T2.B = T1.A)
Is this correct? When i try this I get an error saying T1.A is invalid indentifier.
I know that i can do select * from T1,T2 where T1.A = T2.B
But my use case is very complex. The query (select * from where T2 where T2.B = T1.A) is very complex.
So how do I go ahead with this?
You just need to JOIN the tables:
select *
from T1
inner join T2
on T2.B = T1.A
If you need help learning JOIN syntax, here is a great visual explanation of joins.
I used an INNER JOIN which will return the rows that match between T1 and T2. You might need to use a LEFT JOIN which will return all rows in T1 even if there is not a matching row in T2
If you have another query to select from, then you can use a subquery:
select *
from T1
inner join
(
-- place your query here
select *
from T2
) T2
on T2.B = T1.A
If your subquery is only returning one column, then you could use:
select t1.*, (select t2.col1 from T2 t2 where t2.B = t1.A)
from T1 t1
Unless I'm mistaken, can't you just use JOIN:
select *
from t1
join t2 on t1.field = t2.field
Good luck.
You can do
select *
from T1
inner join T2
on T2.B = T1.A
as other people have said.
However, this "ON" version is preferred only for readability.
You can also go ahead an use
select * from T1,T2 where T1.A = T2.B
The optimizer will figure it out, and do the exact same thing,
as the queries are equivalent.
You can go ahead an use it, as there is nothing wrong with it.
select * from first_table inner join second_table on first_table.X = second_table.Y
select A,T1.B,(select * from where T2 where T2.B = T1.A) FROM T1 .Will this help?
how can i use two or more condition on join?
i want to replace this query with join version of it:
select *
from t1,t2
where t1.a=t2.b and t1.c=t2.d and t1.e=t2.f
how could it be?
This should work
select *
from t1
join t2
on t1.a = t2.b and t1.c = t2.d and t1.e = t2.f
I want get the maximum value for this record. Please help me:
SELECT rest.field1
FROM mastertable AS m
INNER JOIN (
SELECT t1.field1 field1,
t2.field2
FROM table1 AS T1
INNER JOIN table2 AS t2 ON t2.field = t1.field
WHERE t1.field3=MAX(t1.field3)
-- ^^^^^^^^^^^^^^ Help me here.
) AS rest ON rest.field1 = m.field
As you've noticed, the WHERE clause doesn't allow you to use aggregates in it. That's what the HAVING clause is for.
HAVING t1.field3=MAX(t1.field3)
You could use a sub query...
WHERE t1.field3 = (SELECT MAX(st1.field3) FROM table1 AS st1)
But I would actually move this out of the where clause and into the join statement, as an AND for the ON clause.
The correct way to use max in the having clause is by performing a self join first:
select t1.a, t1.b, t1.c
from table1 t1
join table1 t1_max
on t1.id = t1_max.id
group by t1.a, t1.b, t1.c
having t1.date = max(t1_max.date)
The following is how you would join with a subquery:
select t1.a, t1.b, t1.c
from table1 t1
where t1.date = (select max(t1_max.date)
from table1 t1_max
where t1.id = t1_max.id)
Be sure to create a single dataset before using an aggregate when dealing with a multi-table join:
select t1.id, t1.date, t1.a, t1.b, t1.c
into #dataset
from table1 t1
join table2 t2
on t1.id = t2.id
join table2 t3
on t1.id = t3.id
select a, b, c
from #dataset d
join #dataset d_max
on d.id = d_max.id
having d.date = max(d_max.date)
group by a, b, c
Sub query version:
select t1.id, t1.date, t1.a, t1.b, t1.c
into #dataset
from table1 t1
join table2 t2
on t1.id = t2.id
join table2 t3
on t1.id = t3.id
select a, b, c
from #dataset d
where d.date = (select max(d_max.date)
from #dataset d_max
where d.id = d_max.id)
SELECT rest.field1
FROM mastertable as m
INNER JOIN table1 at t1 on t1.field1 = m.field
INNER JOIN table2 at t2 on t2.field = t1.field
WHERE t1.field3 = (SELECT MAX(field3) FROM table1)
yes you need to use a having clause after the Group by clause ,
as the where is just to filter the data on simple parameters ,
but group by followed by a Having statement is the idea to group the data and filter it on basis of some aggregate function......
But its still giving an error message in Query Builder. I am using SqlServerCe 2008.
SELECT Products_Master.ProductName, Order_Products.Quantity, Order_Details.TotalTax, Order_Products.Cost, Order_Details.Discount,
Order_Details.TotalPrice
FROM Order_Products INNER JOIN
Order_Details ON Order_Details.OrderID = Order_Products.OrderID INNER JOIN
Products_Master ON Products_Master.ProductCode = Order_Products.ProductCode
HAVING (Order_Details.OrderID = (SELECT MAX(OrderID) AS Expr1 FROM Order_Details AS mx1))
I replaced WHERE with HAVING as said by #powerlord. But still showing an error.
Error parsing the query. [Token line number = 1, Token line offset = 371, Token in error = SELECT]