I am using Sqlite3. What is the most efficient way to:
Execute a query to join two tables t1 and t2. The query is designed to keep all of the columns from t1, and just add columns from t2. It will either keep all rows (left outer join) or just keep the matching rows.
Using the result, update t1 in the database to look exactly like the results of the query.
Try this:
select a.*, b.hostname, b.mac
from tablea a left join
tableb b on a.ip_address = b.ip
Related
I have 8 tables all with equal number of columns and with a common column. I want to fetch data from all tables in a single query.
My table structure is TABLE1, TABLE2, TABLE3, ..... TABLE 8.
that have columns COLUMNA, COLUMNB... COLUMNE and a COMMON_COLUMN
I need to get data with a where clause where COMMON_COLUMN='X'
I will need all columns from all tables.
I used a query that goes like this..
SELECT TABLE1.*, TABLE2.*, TABLE3.*
FROM TABLE1 T1
LEFT JOIN TABLE2 T2 ON T1.COMMON_COLUMN = T2.COMMON_COLUMN,
LEFT JOIN TABLE3 T3 ON T1.COMMON_COLUMN = T3.COMMON_COLUMN
WHERE T1.COMMON_COLUMN='X' AND T2.COMMON_COLUMN='X' AND T3.COMMON_COLUMN='X'
The above query is not giving any results even if one of the tables do not have any rows. I do not want to use inner join because although the tables have a common column they do not depend on each other and I need data from all tables with a certain common column.
Also, the tables have unequal number of rows.
What am I doing wrong?
correct me if i am wrong - as you do not attach any sample data and desired result
but i assume that you simply need union all tables. You write in the title that tables are independent
SELECT T1.*
FROM TABLE1 T1
WHERE T1.COMMON_COLUMN='X'
UNION ALL
SELECT T2.*
FROM TABLE2 T2
WHERE T2.COMMON_COLUMN='X'
UNION ALL
SELECT T3.*
FROM TABLE3 T3
WHERE T3.COMMON_COLUMN='X'
...
I need to select all data from 2 tables in an sql database.
I searched the site and dried numerous ways but no sucess.
One table has no data but the other is full of it.
If i select each one individually i get good results, but if i use for instance:
select * from relatorio cross join temp
or
select * from relatorio r,temp t
or even:
select t.*, r.* from temp t inner join relatorio r on 1=1
The join works, but none of them shows data.
Can anyone help?
Thanks in advance.
All three select statements in the questions are cross joins.
A cross joins returns data only if both tables have at least one row.
It returns a cartesian product of both tables, meaning that every row in one table will be joined to every row in the other table.
One table has no data but the other is full of it.
Since one of your tables is empty, it will return no results at all. You can think about it as multipling by 0.
Now you have two options: one is to use a full join and the other one is to use left join, in this case both will return the same results, since one table is empty:
select *
from relatorio
left join temp on <join condition> -- assuming temp is the empty table
or
select *
from relatorio
full join temp on <join condition> -- in this case, it doesn't matter what table is empty
If you want to return all matched and umatched rows, use Full Outer Join.The FULL OUTER JOIN keyword returns all rows from the left table (table1) and from the right table (table2).
The FULL OUTER JOIN keyword combines the result of both LEFT and RIGHT joins.
In SQL the FULL OUTER JOIN combines the results of both left and right outer joins and returns all (matched or unmatched) rows from the tables on both sides of the join clause.
SQL FULL OUTER JOIN Syntax:
SELECT column_name(s)
FROM table1
FULL OUTER JOIN table2
ON table1.column_name=table2.column_name;
The SQL CROSS JOIN produces a result set which is the number of rows in the first table multiplied by the number of rows in the second table if no WHERE clause is used along with CROSS JOIN. This kind of result is called as Cartesian Product.
If WHERE clause is used with CROSS JOIN, it functions like an INNER JOIN.
An alternative way of achieving the same result is to use column names separated by commas after SELECT and mentioning the table names involved, after a FROM clause.
CROSS JOIN SYNTAX
SELECT *
FROM table1
CROSS JOIN table2;
I have read a number of posts from SO and I understand the differences between filtering in the where clause and on clause. But most of those examples are filtering on the RIGHT table (when using left join). If I have a query such as below:
select * from tableA A left join tableB B on A.ID = B.ID and A.ID = 20
The return values are not what I expected. I would have thought it first filters the left table and fetches only rows with ID = 20 and then do a left join with tableB.
Of course, this should be technically the same as doing:
select * from tableA A left join table B on A.ID = B.ID where A.ID = 20
But I thought the performance would be better if you could filter the table before doing a join. Can someone enlighten me on how this SQL is processed and help me understand this thoroughly.
A left join follows a simple rule. It keeps all the rows in the first table. The values of columns depend on the on clause. If there is no match, then the corresponding table's columns are NULL -- whether the first or second table.
So, for this query:
select *
from tableA A left join
tableB B
on A.ID = B.ID and A.ID = 20;
All the rows in A are in the result set, regardless of whether or not there is a match. When the id is not 20, then the rows and columns are still taken from A. However, the condition is false so the columns in B are NULL. This is a simple rule. It does not depend on whether the conditions are on the first table or the second table.
For this query:
select *
from tableA A left join
tableB B
on A.ID = B.ID
where A.ID = 20;
The from clause keeps all the rows in A. But then the where clause has its effect. And it filters the rows so on only id 20s are in the result set.
When using a left join:
Filter conditions on the first table go in the where clause.
Filter conditions on subsequent tables go in the on clause.
Where you have from tablea, you could put a subquery like from (select x.* from tablea X where x.value=20) TA
Then refer to TA like you did tablea previously.
Likely the query optimizer would do this for you.
Oracle should have a way to show the query plan. Put "Explain plan" before the sql statement. Look at the plan both ways and see what it does.
In your first SQL statement, A.ID=20 is not being joined to anything technically. Joins are used to connect two separate tables together, with the ON statement joining columns by associating them as keys.
WHERE statements allow the filtering of data by reducing the number of rows returned only where that value can be found under that particular column.
I am trying to run a left join on 2 tables. I do not have a group by and the only where condition i have is on the second table. But, the returned rows are less than the first table. isn't the left join suppose to bring all the data from the first table?
Here is my SQL:
select *
from tbl_a A left join tbl_b B
ON
A.Cnumber=B.Cnumber
and A.CDNUmber=B.CDNumber
and abs(A.duration - B.Duration)<2
and substr(A.text,1,3)||substr(A.text,5,8)||substr(A.text,9,2)=substr(B.text,1,8)
where B.fixed = 'b580'
There are 140,000 records in table A but the result returned is less than 100,000 records. What is the problem and how can I solve it?
As soon as you put a condition in the WHERE clause that references the right table and doesn't accommodate the NULLs that will be produced when the join is unsuccessful, you've transformed it (effectively) back into an INNER JOIN.
Try:
where B.fixed = 'b580' OR B.fixed IS NULL
Or add this condition to the ON clause for the JOIN.
You should add the where clause to the join:
select *
from tbl_a A left join tbl_b B
ON
A.Cnumber=B.Cnumber
and A.CDNUmber=B.CDNumber
and abs(A.duration - B.Duration)<2
and substr(A.text,1,3)||substr(A.text,5,8)||substr(A.text,9,2)=substr(B.text,1,8)
and B.fixed = 'b580'
If you use where statemen all records where b is not existing will not returned.
I am building an application which dynamically generates sql to search for rows of a particular Table (this is the main domain class, like an Employee).
There are three tables Table1, Table2 and Table1Table2Map.
Table1 has a many to many relationship with Table2, and is mapped through Table1Table2Map table. But since Table1 is my main table the relationship is virtually like a one to many.
My app generates a sql which basically gives a result set containing rows from all these tables. The select clause and joins dont change whereas the where clause is generated based on user interaction. In any case I dont want duplicate rows of Table1 in my result set as it is the main table for result display. Right now the query that is getting generated is like this:
select distinct Table1.Id as Id, Table1.Name, Table2.Description from Table1
left outer join Table1Table2Map on (Table1Table2Map.Table1Id = Table1.Id)
left outer join Table2 on (Table2.Id = Table1Table2Map.Table2Id)
For simplicity I have excluded the where clause. The problem is when there are multiple rows in Table2 for Table1 even though I have said distinct of Table1.Id the result set has duplicate rows of Table1 as it has to select all the matching rows in Table2.
To elaborate more, consider that for a row in Table1 with Id = 1 there are two rows in Table1Table2Map (1, 1) and (1, 2) mapping Table1 to two rows in Table2 with ids 1, 2. The above mentioned query returns duplicate rows for this case. Now I want the query to return Table1 row with Id 1 only once. This is because there is only one row in Table2 that is like an active value for the corresponding entry in Table1 (this information is in Mapping table).
Is there a way I can avoid getting duplicate rows of Table1.
I think there is some basic problem in the way I am trying to solve the problem, but I am not able to find out what it is. Thanks in advance.
Try:
left outer join (select distinct YOUR_COLUMNS_HERE ...) SUBQUERY_ALIAS on ...
In other words, don't join directly against the table, join against a sub-query that limits the rows you join against.
You can use GROUP BY on Table1.Id ,and that will get rid off the extra rows. You wouldn't need to worry about any mechanics on join side.
I came up with this solution in a huge query and it this solution didnt effect the query time much.
NOTE : I'm answering this question 3 years after its been asked but this may help someone i believe.
You can re-write your left joins to be outer applies, so that you can use a top 1 and an order by as follows:
select Table1.Id as Id, Table1.Name, Table2.Description
from Table1
outer apply (
select top 1 *
from Table1Table2Map
where (Table1Table2Map.Table1Id = Table1.Id) and Table1Table2Map.IsActive = 1
order by somethingCol
) t1t2
outer apply (
select top 1 *
from Table2
where (Table2.Id = Table1Table2Map.Table2Id)
) t2;
Note that an outer apply without a "top" or an "order by" is exactly equivalent to a left outer join, it just gives you a little more control. (cross apply is equivalent to an inner join).
You can also do something similar using the row_number() function:
select * from (
select distinct Table1.Id as Id, Table1.Name, Table2.Description,
rowNum = row_number() over ( partition by table1.id order by something )
from Table1
left outer join Table1Table2Map on (Table1Table2Map.Table1Id = Table1.Id)
left outer join Table2 on (Table2.Id = Table1Table2Map.Table2Id)
) x
where rowNum = 1;
Most of this doesn't apply if the IsActive flag can narrow down your other tables to one row, but they might come in useful for you.
To elaborate on one point: you said that there is only one "active" row in Table2 per row in Table1. Is that row not marked as active such that you could put it in the where clause? Or is there some magic in the dynamic conditions supplied by the user that determines what's active and what isn't.
If you don't need to select anything from Table2 the solution is relatively simply in that you can use the EXISTS function but since you've put TAble2.Description in the clause I'll assume that's not the case.
Basically what separates the relevant rows in Table2 from the irrelevant ones? Is it an active flag or a dynamic condition? The first row? That's really how you should be removing duplicates.
DISTINCT clauses tend to be overused. That may not be the case here but it sounds like it's possible that you're trying to hack out the results you want with DISTINCT rather than solving the real problem, which is a fairly common problem.
You have to include activity clause into your join (and no need for distinct):
select Table1.Id as Id, Table1.Name, Table2.Description from Table1
left outer join Table1Table2Map on (Table1Table2Map.Table1Id = Table1.Id) and Table1Table2Map.IsActive = 1
left outer join Table2 on (Table2.Id = Table1Table2Map.Table2Id)
If you want to display multiple rows from table2 you will have duplicate data from table1 displayed. If you wanted to you could use an aggregate function (IE Max, Min) on table2, this would eliminate the duplicate rows from table1, but would also hide some of the data from table2.
See also my answer on question #70161 for additional explanation