I am relatively new to SQL and I have the following question. I have the following code:
Select * from table1
LEFT JOIN table2 ON table1.name = table2.name and table1.id = table2.id
LEFT JOIN (SELECT id FROM table2 GROUP BY id) newtable ON table1.id = newtable.id
As both left joins uses data from the same table, is it possible to combine the two joins into one? How would the filters work in this case?
If your goal just to join table2 based on distinct values, then you can use WHERE and GROUP BY:
Select
*
from table1 t1
LEFT JOIN table2 t2
ON t1.name = t2.name and t1.id = t2.id
WHERE t1 id in (SELECT s2.id FROM table2 s2 GROUP BY s2.id)
Related
I am trying to join two tables on two different columns and I was wondering if following two techniques are equivalent, if yes which one is better performance wise?
JOIN with OR (Conditional JOIN)
SELECT *
FROM table1
JOIN TABLE2 ON table1.value = table2.HighValue
OR table1.value = table2.LowValue
Using self Join
SELECT *
FROM TABLE1
JOIN table2 t2 ON table1.value = t2.HighValue
JOIN table2 t3 ON table1.value = t3.LowValue
The two queries are not equivalent, the first is equivalent to
SELECT *
FROM table1
JOIN TABLE2 ON table1.value = table2.HighValue
UNION
SELECT *
FROM table1
JOIN TABLE2 ON table1.value = table2.LowValue
Hi I have Table1 and Table2 in sqlserver. I want the result like in Table3 as can be seen in the image below.
This query gave me the answer finally:
SELECT COALESECE(t1.Label, t2.label) AS label,
t1.Value AS Table1_Value,
t2.Value AS Table2_Value
FROM Table_1 t1
FULL OUTER JOIN Table_2 t2
ON t1.Label = t2.Label
Try the below Query
SELECT COALESCE(T1.LABEL,T2.LABEL) AS LABEL,T1.VALUE,T2.VALUE
FROM TAB1 T1 FULL OUTER JOIN TAB2 T2
ON T1.LABEL=T2.LABEL
Using This query:
SELECT t1.Label,
t1.Value AS Table1_Value,
t2.Value AS Table2_Value
FROM Table1 t1
FULL OUTER JOIN Table2 t2
ON t1.Label = t2.Label
I'm trying to figure out some basic rules in T-SQL.
What I'm trying to achieve here, is to get only the records from Table1 which has a match in Table2 - AND - all records from Table1 where the 'Valid' column has a value of 1 (=true).
Previously I've done this with two selects and a UNION like this:
SELECT T1.*
FROM Table1 T1
INNER JOIN Table2 T2 ON T1.ID = T2.ID
UNION
SELECT T1.*
FROM Table1 T1
WHERE T1.Valid = 1
But isn't there any other way than using multiple selects and UNION to achieve this?
While fiddling, I did the following code bit, which however only works if there's exactly one match in Table2 (otherwise it'll multiply the records by the number of matches in T2).
SELECT T1.*
FROM Table1 T1
INNER JOIN Table2 T2 ON T1.ID = T2.ID
OR T1.Valid = 1
What would be the best way to achieve my goal in terms of performance?
Also please don't hold back on the comments, possible flaws, or explanations of how and why another solution might be better.
assuming that T1.ID and T2.ID is unique or a primary key:
If there are duplicates you may have to write SELECT DISTINCT T1.*. The UNION operator in the orinal selects only distinct values.
this one should do:
SELECT T1.*
FROM Table1 T1
WHERE T1.ID IN ( SELECT T2.ID FROM Table2 T2 WHERE T2.ID IS NOT NULL)
OR T1.Valid = 1
or
SELECT T1.*
FROM Table1 T1
LEFT JOIN Table2 T2 ON T1.ID = T2.ID
WHERE T2.ID IS NOT NULL OR T1.Valid = 1
but i think, the execution plan will be the same at the end.
There is this SQL Statement
SELECT t1.Name
,Count(t2.SubID) Totals -- I don't know how to do it.
FROM Table t1
INNER JOIN Table2 t2 ON t1.ID = t2.SubID
Thanks.
Guessing... change the JOIN and add GROUP BY
SELECT t1.Name
,Count(t2.SubID) AS Totals
FROM Table t1
LEFT OUTER JOIN Table2 t2 ON t1.ID = t2.SubID
GROUP BY t1.Name
The LEFT OUTER JOIN allows you to find count zero rows per t1.Name
SELECT t1.Name, Count(t2.SubID) Totals
FROM Table t1
INNER JOIN Table2 t2 ON t1.ID = t2.SubID
GROUP BY t1.Name
Table 1 2 columns: ID, Name
Table 2 2 columns: ID, Name
What is a query to show names from Table 1 that are not in table 2? So filtering out all the names in table 1 that are in table 2 gives the result query. Filtering is on ID not name.
Select * from table1
left join table2 on table1.id = table2.id
where table2.id is null
This should perform better than the left join...is null version. See here and here for comparisons.
select t1.id, t1.name
from table1 t1
where not exists(select null from table2 t2 where t2.id = t1.id)
Use this query
select
t1.*
from table1 t1
left outer join table2 t2
on t1.id=t2.id
where t2.id is null
this works by joining everything in t1 to whatever exists in t2. the where clause filters out all of the records that don't exist in t2.
SELECT Table1.ID, Table1.Name, Table2.ID
FROM Table1 LEFT OUTER JOIN Table2 ON Table1.ID = Table2.ID
WHERE Table2.ID IS NULL
I think that should do it.
Try like this:
select t1.*
from table1 as t1
where t1.id not in
(select distinct t2.id from table2 as t2);