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.
Related
Consider the following two queries:
select *
from
table1 t1
left join
table2 t2
on t1.Id = t2.t1Id and (t1.Status = 1 or t2.Id is not null)
And this one
select *
from
table1 t1
left join
table2 t2
on t1.Id = t2.t1Id
where
t1.Status = 1 or t2.Id is not null
The first one runs in 2 seconds. The second one in 2 minutes. Shouldn't the execution plan be the same?
The query plans are different because the queries (and results) are different.
You're using a LEFT JOIN, so the first query will return rows with NULL values where not in table 2.
The second query will not return those rows.
If it was an INNER JOIN, they would essentially be the same query.
Here the Below Query Returns all the "Table1" results with additional matching Columns based on the "ON Clause" condition.
select * from table1 t1
left join table2 t2
on t1.Id = t2.t1Id and (t1.Status = 1 or t2.Id is not null)
Now, the below query matches the 2 tables and returns the rows based on the ON Clause and an additional WHERE Clause filters the Rows again based on the Condition.
select * from
table1 t1
left join table2 t2 on t1.Id = t2.t1Id
where t1.Status = 1 or t2.Id is not null
Here, Even though we used LEFT JOIN But in this case it acts like an INNER JOIN
So, Here Both the Queries produce Different Result Sets. The Execution Plan Also Vary which results in Different Execution Time.
The best way to deal with an OR is to eliminate it (if possible) or break it into smaller queries. Breaking a short and simple query into a longer, more drawn-out query may not seem elegant, but when dealing with OR problems, it is often the best choice:
select *
from table1 t1
left join table2 t2 t1.Id = t2.t1Id
where t1.Status = 1
union all
select *
from table1 t1
left join table2 t2 t1.Id = t2.t1Id
where t2.Id is not null
You can read more in this article:
https://www.sqlshack.com/query-optimization-techniques-in-sql-server-tips-and-tricks/
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)
I have two tables t1 and t2.
t1 having 10k records and t2 having 2k records. The 2k records of t2 is present in t1.
I wanted the 8k different records from t1 which is not present in t2.
I'm doing this as below:
select id, second_telphon from t1
except
select id, second_telphon from t2
However, I'm still getting all the 10k records. Is "except" keyword not working?
how can I achieve this?
you can perform a Join to get the unique data from the tables .
like the tables t1 & t2 both you cna perform left or right join .
example:
SELECT T1.*
FROM T1
WHERE NOT EXISTS(SELECT NULL
FROM T2
WHERE T1.ID = T2.ID
AND T1.Date = T2.Date
AND T1.Hour = T2.Hour)
OR .
SELECT T1.*
FROM T1
LEFT JOIN T2
ON T1.ID = T2.ID
AND T1.Date = T2.Date
AND T1.Hour = T2.Hour
WHERE T2.ID IS NULL
Try this:
SELECT *
FROM T1
WHERE NOT EXISTS(SELECT id,second_telphon FROM t2)
If ID is a unique value, Try this also:
SELECT *
FROM T1
WHERE ID NOT IN(SELECT ID FROM t2)
You could try a union, followed by an aggregation to restrict to those records in the first table which were not duplicated by the second table:
SELECT id, second_telphon
FROM
(
SELECT id, second_telphon FROM t1
UNION ALL
SELECT id, second_telphon FROM t2
) t
GROUP BY id, second_telphon
HAVING COUNT(*) = 1;
If a record, being defined as an id, second_telphon pair, has a record count of only one after the union, it implies that this record was unique to the first table.
Just do left join
select t1.id,t1.second_telphon from t1
left join t2 on
t1.id = t2.id
and t1.second_telphon =t2.second_telphon
where t2.id is null
I'm wondering if I can just do an inner join as kind of a where clause by itself. Or if I use a field from the joined table in my where clause, if it's redundant.
select * from T1 inner join T2 on T1.id = T2.id where T2.z is not null
Is the "T2.z is null" part redundant if all I want returned are records in T1 where the same id exists in T2?
For one thing, select * from t1 inner join t2 [...] will not return records in t1 - it will return all the columns of t1 and t2. You could fix that by selecting specifically the columns in t1 - don't select *.
Then, if there are many rows in t2 with the same t2.id, matching a given t1.id, you will get a whole bunch of rows in the result for that one row in the input t1. So you will not always "reduce" the result set.
It seems what you want can be achieved with the in operator, something like
select * from t1 where t1.id in (select id from t2);
This is equivalent to the following modification of your query. You do not need a where clause for this to work:
select t1.* from t1 inner join (select distinct id from t2) b on t1.id = b.id;
In the following query,
select t1.* from T1 inner join T2 on T1.id = T2.id where T2.z is NOT null
The WHERE condition is redundant, assuming that T2.Z is a NOT NULL column.
That would leave you with this:
select t1.* from T1 inner join T2 on T1.id = T2.id
, which is a little odd because, in a normally designed database, either T1.id or T2.id would be the primary key of its table.
If T1.id is the primary key of T1, then your query is going to return duplicates -- each T1 row will be repeated once for each child that exists in T2.
If T2.id is the primary key of T2, then you should not need to join to T2 at all, because every possible T1.id value must exist in T2.id, because of the FOREIGN KEY relationship that (should) exist. In that case, you could have written:
select t1.* from T1 WHERE T1.id is not null;
So, the answer to your question is that you do not need to reference the tables outside of the join condition in order for the join to be applied. But something seems a little off about the approach.
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);