I have two tables with 3.5 million rows of data. I am creating a left join between the two to create a new view.
Code 1:
SELECT t1.c1,t1.c2,t2.c3,t2.c4
from table1 as t1
left join table2 as t2
on t1.Location=t2.Location and t1.OrderNumber=t2.OrderNumber and t1.Customer=t2.Customer
Code 2:
SELECT t1.c1,t1.c2,t2.c3,t2.c4
from table1 as t1
left join table2 as t2
on t1.OrderNumber=t2.OrderNumber
Both snippets of code give the same desired result as the Order number field in table 2 has only unique values.
Is it better to give more fields to JOIN compared to only one?
SELECT t1.c1,t1.c2,t2.c3,t2.c4
from table1 as t1
left join table2 as t2
on t1.Location = t2.Location
and t1.OrderNumber = t2.OrderNumber
and t1.Customer = t2.Customer
If OrderNumber is the PK of either table then adding additional fields will not change the results and it will not improve performance unless an index as not present on the other side.
If Order number field in table 2 has only unique values it would not change the query. If it is a PK or has a unique constraint/index then addition fields would not help unless what Table2.OrderNumber was joined to was not indexed.
Related
I am trying to write an SQL query that will return Table1, which has 10 columns. This table consists of a primary key id, 4 foreign key Id columns, and 5 other columns that I want to return but not change. The goal is to do a join to replace the foreign key Ids with their descriptions that are held in other tables.
Here is one attempt with the first FK Id:
Select * from Table1 t1
left join Table2 t2
on t1.BranchId = t2.BranchId;
This left join returns the description from table2, but does not replace it.
Here is another with the first FK Id:
Select t2.BranchName from Table1 t1
left join Table2 t2
on t1.BranchId = t2.BranchId;
This returns the name I want, but does not return table1 fully.
For the sake of an example you could pretend that OtherName3, OtherName4, OtherName5 are in tables Table3, Table4, Table5, respectively.
This may seem trivial for experienced SQL devs, but I am having a hard time figuring out the syntax.
Thanks!
I'm not sure what you mean by replace it.
I think you just need to list out all the columns you want:
Select t1.col1, t1.col2, t1.col3, . . .,
t2.name
from Table1 t1 left join
Table2 t2
on t1.BranchId = t2.BranchId;
I don't know what you mean by 'replace' but you just need to qualify what columns from which table you want. That goes for all tables you are joined to, especially if they have the same column name in multiple tables. I put junk columns in since I don't know your tables but you should get the general idea.
Select t2.BranchName, t1.BranchId, t1.Name, t1.Amount, t2.BranchLocation from Table1 t1
left join Table2 t2
on t1.BranchId = t2.BranchId;
I think this is what you are looking for:
select t1.*, t2.BranchName from Table1 t1
left join Table2 t2
on t1.BranchId = t2.BranchId;
Return Table1 fully (all columns) and only the description (BranchName) from Table2.
If using SQL Server, see all syntax options for the SELECT clause here:
https://msdn.microsoft.com/en-us/library/ms176104.aspx
How can I join three or four SQL tables that DO NOT have an equal amount of rows while ensuring that there are no duplicates of a primary/foreign key?
Structure:
Table1: id, first_name, last_name, email
Table2: id (independent of id in table 1), name, location, table1_id, table2_id
Table3: id, name, location
I want all of the data from table 1, then all of the data from table 2 corresponding with the table1_id without duplicates.
Kind of tricky for a new guy...
Not sure what do you want to do with Table3.
A LEFT JOIN returns all records from the LEFT table, and the matched records from the right table. If there is no match (from the right side), then the result is NULL.
So per example:
SELECT * FROM Table1 AS t
LEFT JOIN Table2 AS tt
ON t.id = tt.id
The LEFT table refers to the table statement before the LEFT JOIN, and the RIGHT table refers to the table statement after the LEFT JOIN. If you want to add in Table3 as well, use the same logic:
SELECT * FROM Table1 AS t
LEFT JOIN Table2 AS tt
ON t.id = tt.id
LEFT JOIN Table3 AS ttt
ON t.id = ttt.id
Note, that I use alias names for the tables (by using AS), so that I can more easily refer to a specific table. For example, t refers to Table1, tt refers to Table2, and ttt refers to Table3.
Joins are often used in SQL, therefore it is useful to look into: INNER JOIN, RIGHT JOIN, FULL JOIN, and SELF JOIN, as well.
Hope this helps.
Good luck with learning!
You will want to use an LEFT JOIN
SELECT * FROM table1 LEFT JOIN table2 ON Table1.ID = Table2.table1_id
I have been working on this problem for 3 months now and gave up once or twice. Yes, I am a novice. I created 3 tables with data. Table 1 has a letter and number. Table 2 has a name, letter and number. Table 3 has the end result. I want to compare the T1 and T2. If the name and number in T1 matches a name and number in table 2. I want the result to in T3 to include name, letter and number. This is what I have so far but it is not working.
SELECT * FROM T1 and SELECT * FROM T2
WHEN
TABLE T1(letter) && TABLE T2(letter)
AND
TABLE T1(number) && TABLE T2(letter)
INSERT INTO TABLE T3 (name,letter,number)
What you need is to do an inner join of the first and second tables based on the attributes name and letter.
SELECT T2.name, T2.letter, T2.number
FROM T1
INNER JOIN T2
ON T1.letter=T2.letter AND T1.number=T2.number;
For more details, you can refer http://www.w3schools.com/sql/sql_join.asp
To expand upon Dinesh's answer,
You'll need an inner join for this. Inner joins gives you rows that match the columns in both tables you've specified.
You can then combine it into an insert statement to put it into your T3 table. So this is one complete SQL statement:
INSERT INTO T3
SELECT T2.name, T2.letter, T2.number
FROM T2
INNER JOIN T1
ON T2.letter = T1.letter
AND T2.number = T1.number;
As a side note, there's also left joins and right joins (and heaps more). Think of Left, Inner, Right joins as two circles in a venn diagram.
I'm pretty amateur when it comes to writing SQL, so maybe this is an easy one for someone?
I have two tables as follows:
Table1 - DeviceName, DeviceID, AlternateID
MyPhone, 333, AAA
HerPhone, 444, CCC
Table2 - PhoneID, ProgramName
333, AngryBirds
CCC, Evernote
As you can see, Table2 uses two different PhoneID types from Table1 (DeviceID and AlternateID). I'm looking a sql statement that will result in output like:
MyPhone, AngryBirds
HerPhone, Evernote
Appreciate any assistance.
Cheers,
Mark
SELECT deviceName, programName
FROM table2 t2
JOIN table1 t1
ON(t1.DeviceID=t2.PhoneID OR t1.AlternateID=t2.PhoneID)
or (less readable but shorter)
SELECT deviceName, programName
FROM table2 t2
JOIN table1 t1
ON(t2.PhoneID IN (t1.DeviceID, t1.AlternateID))
Still, if DeviceIDs and AlternateIDs are from the same set, you should consider refactoring the database: What if a device could have multiple valid IDs, not just two?
What if both 333 and AAA are defined in Table 2? I'm going to assume you only want want one row returned per DeviceName, which means you need to join to Table 2 twice using outer joins:
select DeviceName,
t2a.ProgramName,
t2b.ProgramName as AltProgramName
from table1 t1
left outer join table2 t2a
on t1.DeviceID = t2a.PhoneID
left outer join table2 t2b
on t1.AlternateID = t2b.PhoneID
If you want to list only one ProgramName, and can establish a priority as to which to use in case both are present, you could do the following: (assuming DeviceID trumps AlternateID)
select DeviceName,
coalesce(t2a.ProgramName, t2b.ProgramName) as AltProgramName
from table1 t1
left outer join table2 t2a
on t1.DeviceID = t2a.PhoneID
left outer join table2 t2b
on t1.AlternateID = t2b.PhoneID
If you want the 1st program column to always contain a value, and only list a value in the 2nd column if both are present, then
select DeviceName,
coalesce(t2a.ProgramName, t2b.ProgramName) as ProgramName1,
case when t2a.ProgramName is not null then t2b.ProgramName end as ProgramName2
from table1 t1
left outer join table2 t2a
on t1.DeviceID = t2a.PhoneID
left outer join table2 t2b
on t1.AlternateID = t2b.PhoneID
EDIT
I assumed the query was being used for reporting purposes, in which case you most likely only want one row per device.
I just realized why the OP might want multiple rows returned per device. If the query is used to define a view for looking up devices by either name, then multiple rows are desired.
Jan's answer works perfectly for the lookup view. However, it may or may not perform well depending on the database engine's query optimizer and the size of the tables. If the tables are very large, then you are going to want an indexed lookup. The join with an OR condition may preclude an indexed lookup on some systems. An equivalent query using UNION ALL may support an indexed lookup on more systems.
select DeviceName,
ProgramName
from table1 t1
join table2 t2
on t1.DeviceID = t2.PhoneID
union all
select DeviceName,
ProgramName
from table1 t1
join table2 t2
on t1.AlternateID = t2.PhoneID
I am a little confused as to how to approach this SQL query.
I have two tables (equal number of records), and I would like to return a column with which is the division between the two.
In other words, here is my not-working-correctly query:
SELECT( (SELECT v FROM Table1) / (SELECT DotProduct FROM Table2) );
How would I do this? All I want it a column where each row equals the same row in Table1 divided by the same row in Table2. The resulting table should have the same number of rows, but I am getting something with a lot more rows than the original two tables.
I am at a complete loss. Any advice?
It sounds like you have some kind of key between the two tables. You need an Inner Join:
select t1.v / t2.DotProduct
from Table1 as t1
inner join Table2 as t2
on t1.ForeignKey = t2.PrimaryKey
Should work. Just make sure you watch out for division by zero errors.
You didn't specify the full table structure so I will assume a common ID column to link rows in the tables.
SELECT table1.v/table2.DotProduct
FROM Table1 INNER JOIN Table2
ON (Table1.ID=Table2.ID)
You need to do a JOIN on the tables and divide the columns you want.
SELECT (Table1.v / Table2.DotProduct) FROM Table1 JOIN Table2 ON something
You need to substitue something to tell SQL how to match up the rows:
Something like: Table1.id = Table2.id
In case your fileds are both integers you need to do this to avoid integer math:
select t1.v / (t2.DotProduct*1.00)
from Table1 as t1
inner join Table2 as t2
on t1.ForeignKey = t2.PrimaryKey
If you have multiple values in table2 relating to values in table1 you need to specify which to use -here I chose the largest one.
select t1.v / (max(t2.DotProduct)*1.00)
from Table1 as t1
inner join Table2 as t2
on t1.ForeignKey = t2.PrimaryKey
Group By t1.v