How to decide the tables which is left and which is right while querying it using joins?Is there any Thumb Rule.
"left" is that table which appears first after the FROM clause when reading from left to right
example:
select t1.* ,t2.* from table1 t1 left join table2 t2 on t1.id=t2.id
here table1 is left table
Related
I tried to join these 2 tables with ALTERNATE_ID's using substring in left outer join but it doesn't work. Can someone suggest me how to do it.
Here Table1 is master table,
SELECT
tab1.USER_NAME,
tab2.ALTERNATE_ID as 'Contract_no'
FROM TABLE1 tab1
LEFT OUTER JOIN TABLE2 tab2 ON Table2.ALTERNATE_ID = SUBSTRING(tab1.ALTERNATE_ID,0,CHARINDEX('/',tab1.ALTERNATE_ID)
TABLE1:
ALTERNATE_ID 100-0000053-001/0001
TABLE2
ALTERNATE_ID 100-0000053-001
You have to use substring from first char to position of \ - 1
But if you have to join based on part of column, then you should created index on the substring part.
SELECT
tab1.USER_NAME,
tab2.ALTERNATE_ID as 'Contract_no'
FROM TABLE1 tab1
LEFT OUTER JOIN TABLE2 tab2
ON Table2.ALTERNATE_ID
=
SUBSTRING(tab1.ALTERNATE_ID,1,CHARINDEX('/',tab1.ALTERNATE_ID-1)
You can also use SUBSTRING_INDEX to split the string like this:
SELECT tab1.USER_NAME, tab2.ALTERNATE_ID as 'Contract_no'
FROM TABLE1 tab1
LEFT OUTER JOIN TABLE2 tab2
ON Table2.ALTERNATE_ID = SUBSTRING_INDEX(tab1.ALTERNATE_ID, "/", 1);
You can use OUTER APPLY in the following way:
SELECT
tab1.USER_NAME,
tab2.ALTERNATE_ID as 'Contract_no'
FROM TABLE1 tab1
OUTER APPLY (Select ALTERNATE_ID
From TABLE2
Where ALTERNATE_ID=SUBSTRING(tab1.ALTERNATE_ID,1,CHARINDEX('/',tab1.ALTERNATE_ID)-1)) As tab2
If you use PATINDEX, then it's easy to get the position of the digit before the slash.
SELECT *
FROM TABLE1 t1
LEFT JOIN TABLE2 t2
ON t2.ALTERNATE_ID = LEFT(t1.ALTERNATE_ID, PATINDEX('%[0-9]/%', t1.ALTERNATE_ID))
Or looking at it differently, the one in Table1 is like the one in Table2
SELECT *
FROM TABLE1 t1
LEFT JOIN TABLE2 t2
ON t1.ALTERNATE_ID LIKE t2.ALTERNATE_ID+'/%'
Demo on db<>fiddle here
I work with DB / SQL almost on a daily basis and the more I work with sql, the more I'm the opinion that there is no reason to use a right join or a full outer join.
Let's assume we have two tables: table1 and table2. Either I want to receive additional information for the rows in table1 so I can use an inner join on table2 and if I want to keep the original rows if there is no match, I use the left join then:
In case I have to add additional information to table 2, I can do the same and left join table 2 to table on. So I do not see a reason why I should ever use a right join. Is there any use case where you can not use a left join for a right join?
I also wondered if I would ever need a full outer join. Why would you join two tables and keep the rows that do not match of BOTH tables? We you could also achieve this by using two left joins.
Why would you join two tables and keep the rows that do not match of BOTH tables?
The full join has cases where it is useful.One of them is comparing two tables for differences like XOR between tables:
SELECT *
FROM t1
FULL JOIN t2
ON t1.id = t2.id
WHERE t1.id IS NULL
OR t2.id IS NULL;
Example:
t1.id ... t2.id
1 NULL
NULL 2
you could also achieve this by using two left joins.
Yes you could:
SELECT t1.*, t2.*
FROM t1
LEFT JOIN t2
ON t1.id = t2.id
WHERE t2.id IS NULL
UNION ALL
SELECT t1.*, t2.*
FROM t2
LEFT JOIN t1
ON t1.id = t2.id
WHERE t1.id IS NULL;
Some SQL dialects does not support FULL OUTER JOIN and we emulate it that way.
Related: How to do a FULL OUTER JOIN in MySQL?
On the other hand RIGHT JOIN is useful when you have to join more than 2 tables:
SELECT *
FROM t1
JOIN t2
...
RIGHT JOIN t3
...
Of course you could argue that you could rewrite it to correspodning form either by changing join order or using subqueries(inline views). From developer perspective it is always good to have tools(even if you don't have to use them)
This is a very simple question but i couldn't get the result for the life of me.
Where is the "leftness" or "rightness" of a table determined in SQL?
so if have to tables, t1 and t2 with this query:
SELECT * FROM t1 LEFT JOIN t2 ON t1.id = t2.id
I'm certain t1 is on the left but what if i change the order of the ON statement like so:
SELECT * FROM t1 LEFT JOIN t2 ON t2.id = t1.id
is t2 now on the left or is t1 still on the left because it is on the left of LEFT?
i appreciate the time anyone takes to answer this simple question.
n.b. my question is not about the difference between LEFT and RIGHT joins
The order of a table name matters in Join (LEFT, RIGHT), Not the order of join condition.
So here, your both queries are same.
Left join or Left outer join requires table name with the condition "t1 LEFT JOIN t2". Here, t1 is left joined with t2 meaning all elements of t1 and matching rows on t2.
The LEFT JOIN keyword returns all records from the left table (table1), and the matched records from the right table (table2). The result is NULL from the right side, if there is no match.
Similarly
The RIGHT JOIN keyword returns all records from the right table (table2), and the matched records from the left table (table1). The result is NULL from the left side, when there is no match.
Putting in your terms the LEFTness or RIGHTness of a table is determined at
FROM table1 LEFT/RIGHT JOIN table2.
Whatever table is before the keyword LEFT join is considered Left table and similarly whatever table is after keyword RIGHT join is considered as Right table. For more info you can refer SQL-Joins
I've got 3 tables that I want to join and filter on some conditions.
I've first wrote this query:
select * from table1 t1
left join (select * from table2 where table2.fieldX=...) t2
on t1.id_12=t2.id_12
left join table3
on t2.id_23=t3.id_23
where t1.fieldY=...
Then I wanted to make it looks like more canonical by rewritting it like that:
select * from table1 t1
left join table2 t2
on t1.id_12=t2.id_12
left join table3
on t2.id_23=t3.id_23
where table2.fieldX=...
and t1.fieldY=...
But it does not give the same result.
I dont't understand why...
Do you?
Thanks in advance.
When you put table2.fieldX=... in the where clause you eliminate all rows from table1 that do not have a corresponding row in table2. Effectively you are changing the left join into an inner join.
Instead, you can apply the table2 filter in the join itself:
SELECT
*
FROM
Table1 t1
LEFT JOIN Table2 t2 ON t1.id_12 = t2.id_12 AND t2.fieldX = ...
LEFT JOIN Table3 t3 ON t2.id_23 = t3.id_23
WHERE
t1.fieldY = ...
Did you add the inner select where on the second query?
SELECT *
FROM table1 t1
LEFT JOIN table2 t2
on t1.id_12=t2.id_12
LEFT JOIN table3
on t2.id_23=t3.id_23
WHERE t1.fieldY=...
and t2.fieldX=...
I have this query
select *
from alldistros t1
LEFT join origin t2 on t1.name=t2.name
LEFT join desktop t3 on t2.name=t3.name
LEFT join beginnerdistributions t4 on t3.name=t4.name
it add on all my tables. But now when I want to select the name field (which is in all of them) I can't show it. It's just blank when I call it. And I would think so since there's more than 1 columns of the same name.
What can I do to fix this?
Just a plain join won't work, since it removes some of the fields that does not have the properties in the other tables.
You can use the 'AS' keyword to name a column. For instance:
select t1.name AS DistroName, t2.name AS OriginName, t3.name AS DesktopName
from alldistros t1
LEFT join origin t2 on t1.name=t2.name
LEFT join desktop t3 on t2.name=t3.name
LEFT join beginnerdistributions t4 on t3.name=t4.name
select
t1.name as t1_name,
t2.name as t2_name,
t3.name as t3_name
from alldistros t1
LEFT join origin t2 on t1.name=t2.name
LEFT join desktop t3 on t2.name=t3.name
LEFT join beginnerdistributions t4 on t3.name=t4.name
Not sure if it's Oracle only, but USING can do this for you for ad-hoc queries:
SELECT *
FROM TABLEA
JOIN TABLEB USING (NAME)
This will only return one NAME column from the SELECT *.