Ok so i was learning sql joins and was curious to try all joins on the following table:
Table name Demo1:
A
1
1
1
1
1
Table name Demo2:
B
1
1
1
1
1
To my amazement no matter which join i apply i end up with same 25 entries. I am sure about cross join since it gives all combination but what about the other joins how are they returning the same answers for these two tables.
How join statement work is it pick up all entries from the first table
the for every entry, it pick all entries from the second table that is sastified by the on condition.
Hence, the number of result in this case = number of records in A * number of records in B = 25.
Related
I'm trying to join two tables and select certain columns to display in the output including a 'flag' if a certain transaction amount is greater than or equal to 100. The flag would return a 1 if it is, else null.
I thought I could achieve this using a CASE in my SELECT but it only returns one record every time since it returns the first record that meets this condition. How do I just create this 'FLAG' column during my join easily?
SELECT payment_id, amount, type,
CASE
WHEN amount >= 100 THEN 1
ELSE NULL
END AS flag
FROM trans JOIN customers ON (user_id = cust_id)
JOIN bank ON (trans.bank = bank.id)
WHERE (error is false)
I expect an output such as:
payment_id amount type flag
1 81 3 NULL
2 104 2 1
3 150 2 1
4 234 1 1
However, I'm only getting the first record such as:
payment_id amount type flag
2 104 2 1
I tried your table structure in my local and it is working perfectly.
I need one thing from you is in which table you are having error column.
If I comment where condition then it is working fine.
If you're getting fewer rows than you expect, it's either due to:
Join condition
You're doing a INNER joins to the customers and bank tables. If you have 4 source rows in your trans table, but only one row that matches in your customers table (condition user_id = cust_id), then you will only have one row returned.
The same goes for the subsequent join to your bank table. If there you somehow have a transaction that references a bank which is not defined in the bank table, then you won't see a record for this row.
WHERE clause
Obviously you won't see any rows that don't meet the conditions specified here.
It's probably #1 -- check to see if the rows with payment_id IN (1,3,4) have corresponding user id values in the user table and corresponding bank id values in the banks table.
SELECT column_1 FROM table_1,table_2;
When I ran this on my database it returned huge number of rows with duplicate column_1 values. I could not understand why I got these results. Please explain what this query does.
it gives you a cross product from table 1 and table 2
In more layman's terms, it means that for each record in Table A, you get every record from Table B (all possible combinations).
TableA with 3 records and Table B with 3 records gives 9 total records in the result:
TableA-1/B-1
TableA-1/B-2
TableA-1/B-3
TableA-2/B-1
TableA-2/B-2
TableA-2/B-3
TableA-3/B-1
TableA-3/B-2
TableA-3/B-3
Often used as a basis for Cartesian Queries (which themselves are the means to generate, say, a list of future dates based on a recurrence schedule: give me all possible results for the next 6 months, then restrict that set to those whose factor matches my day of the week)
This is 'valid' way of cross joining two tables; it is not the preferred way though. Cross Join would be much clearer. An on condition would then be helpful to limit results,
Imagine that i have 3 friends named Jhon, Ana, Nick; then i have in the other table 2 are T-shirts a red and a yellow and i wanna know witch is from.
So in the query being tableA:Friends and tableB:Tshirts returns:
1|JHON | t-shirt_YELLOW
2|JHON | t-shirt_RED
3|ANA | t-shirt_YELLOW
4|ANA | t-shirt_RED
5|NICK | t-shirt_YELLOW
6|NICK | t-shirt_RED
As you see this join has no relational logic between friends and Tshirts so by evaluating all the posible combination generates what you call duplicates.
I have the following situation. I have a table with all info of article. I will like to compare the same column with it self. because I have multiple type of article. Single product and Master product. the only way that I have to differences it, is by SKU. for example.
ID | SKU
1 | 11111
2 | 11112
3 | 11113
4 | 11113-5
5 | 11113-8
6 | 11114
7 | 11115
8 | 11115-1-W
9 | 11115-2
10 | 11116
I only want to list or / and count only the sku that are full unique. follow th example the sku that are unique and no have variant are (ID = 1, 2, 6 and 10) I will want to create a query where if 11113 are again on the column not cout it. so in total I will be 4 unique sku and not "6 (on total)". Please let me know. if this are possible.
Assuming the length of master SKUs are 5 characters, try this:
select a.*
from mytable a
left join mytable b on b.sku like concat(a.sku, '%')
where length(a.sku) = 5
and b.sku is null
This query joins master SKUs to child ones, but filters out successful joins - leaving only solitary master SKUs.
You can do this by grouping and counting the unique rows.
First, we will need to take your table and add a new column, MasterSKU. This will be the first five characters of the SKU column. Once we have the MasterSKU, we can then GROUP BY it. This will bundle together all of the rows having the same MasterSKU. Once we are grouping we get access to aggregate functions like COUNT(). We will use that function to count the number of rows for each MasterSKU. Then, we will filter out any rows that have a COUNT() over 1. That will leave you with only the unique rows remaining.
Take that unique list and LEFT JOIN it back into your original table to grab the IDs.
SELECT ID, A.MasterSKU
FROM (
SELECT
MasterSKU = SUBSTRING(SKU,1,5),
MasterSKUCount = COUNT(*)
FROM MyTable
GROUP BY SUBSTRING(SKU,1,5)
HAVING COUNT(*) = 1
) AS A
LEFT JOIN (
SELECT
ID,
MasterSKU = SUBSTRING(SKU,1,5)
FROM MyTable
) AS B
ON A.MasterSKU = B.MasterSKU
Now one thing I noticed from you example. The original SKU column really looks like three columns in one. We have multiple values being joined with hypens.
11115-1-W
There may be a reason for it, but most likely this violates first normal form and will make the database hard to query. It's part of the reason why such a complicated query is needed. If the SKU column really represents multiple things then we may want to consider breaking it out into MasterSKU, Version, and Color or whatever each hyphen represents.
I have two tables named LocalVSDB and TemporaryVSDB. Both tables have the same columns:
LocalVSDB: msisdn,activateDate
TemporaryVSDB: msisdn,activateDate
But both tables also have duplicate rows for MSIDSN
I need to join these two tables. My intended result looks like this:
MSISDN LocalActivateDate TemporaryActivateDate Datediff
60103820251 2013-12-14 2013-10-05 70
601111000254 2013-12-14 2013-10-05 70
601111000254 2013-12-18 2013-09-10 80
But, since there are duplicate MSIDSNs, I am getting duplicate rows when I join. For example there are 6 rows for certain MSISDN in each table so when I am joining I am getting total 36 rows for that MSISDN.
I am joining using the following query:
SELECT t.msisdn,t.activateDate AS VSDB_Activate_Date,
l.activateDate AS Local_Activate_Date,
DATEDIFF(D,l.activateDate,t.activateDate) AS date_Diff
FROM temporaryVSDB2 t
INNER JOIN LocalVSDB l ON t.msisdn = l.msisdn
WHERE t.activateDate > l.activateDate
Please help me how can I get 6 rows for 6 MSISDN?
Thanks in advance.
The problem is:
where t.activateDate > l.activateDate
That means one row in table one can join to all six rows in table two. You either need to change this to an = or just get a single row from the second table based on certain criteria.
SELECT m.MSIDN, m.ActiveDate, t.ActiveDate, DATEDIFF(DAY, m.ActiveDate, t.ActiveDate) Duration
FROM LocalVSDB m
OUTER APPLY
(
SELECT TOP 1 d.MSIDN, d.ActiveDate
FROM TemporaryVSDB d
WHERE d.ActiveDate > m.ActiveDate
ORDER BY d.ActiveDate
) t
This would find the nearest partner record and duration (the last record will have a null partner though)
You can use your own query adding group by clause provided msidn and activateDate produce unique row.
SELECT t.msisdn,t.activateDate AS VSDB_Activate_Date,
l.activateDate AS Local_Activate_Date,
DATEDIFF(D,l.activateDate,t.activateDate) AS date_Diff
FROM temporaryVSDB2 t INNER JOIN LocalVSDB l ON t.msisdn = l.msisdn
WHERE t.activateDate > l.activateDate
group by t.msisdn, t.activateDate
When I make this sql statement I get 6 of the same record returned. So if I expect to get 2 records returned, I get six of each record back so that is 12 in total.
SELECT
ce2.*
FROM customerentry ce, customerentrytrace cet, customerentry ce2
WHERE ce.accountid = 1
AND ce.companyid = 1
AND ce.accountid=cet.accountid
AND ce.accountid=ce2.accountid
AND ce.companyid=cet.companyid
AND ce.companyid=ce2.companyid
AND cet.documentno = '2012Faktura1'
AND cet.documenttype = 1
AND ce2.documentno = cet.offsetdocumentno
AND ce2.documenttype = cet.offsetdocumenttype
ORDER BY created;
I know that I can solve it by adding distinct, but I would like to know why I get 6 of the same record returned. Anyone who can help me?
Since we have no idea about your table structure probably there are some columns that are related 1 to n items and you haven't handled them in the WHERE section of your query.
As an extra measure you can focus on your data needs and add a GROUP BY section before your ORDER section.
You are using an INNER JOIN, so for example there are two entries in table cet matching your where clause for combining table ce and cet, giving you 2 entries/entry of table ce.
Thinking this further you can see that if there are 3 entries in table ce2 matching the where clause for combining table cet and ce2 you get 3 entries/entry of table cet.
Which makes 6 entries per entry of table ce in total, giving you 12 entries in total even if you have only 2 entries in table ce.
So think again about what join could be the right for your desired solution.
Here a link for some more explanation: Short explanation of joins
Problem might be because you have not properly joined tables. Please read about JOIN
SELECT ce2.*
FROM customerentry ce INNER JOIN customerentrytrace cet ON ce.accountid=cet.accountid AND ce.companyid=cet.companyid,
INNER JOIN customerentry ce2 ON ce.accountid=ce2.accountid AND ce.companyid=ce2.companyid AND ce2.documentno = cet.offsetdocumentno AND ce2.documenttype = cet.offsetdocumenttype
WHERE ce.accountid = 1
AND ce.companyid = 1
AND cet.documentno = '2012Faktura1'
AND cet.documenttype = 1
ORDER BY created;