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Join vs. sub-query

I am an old-school MySQL user and have always preferred JOIN over sub-query. But nowadays everyone uses sub-query and I hate it, I don't know why.

I lack the theoretical knowledge to judge for myself if there is any difference. Is a sub-query as good as a JOIN and therefore there is nothing to worry about?

What is the syntax for an inner join in LINQ to SQL?

I'm writing a linq to sql statement & I'm just after the standard syntax for a normal inner join with an 'on' clause in C#.

ie how do you represent this in LINQ to SQL?:

select * from table1 
inner join table2 on table1.field = table2.field

EDIT: Real query to get all contacts for a dealer:

select DealerContact.*
from Dealer 
    inner join DealerContact on Dealer.DealerID = DealerContact.DealerID

Difference between INNER and OUTER joins

What is the difference between INNER JOIN and OUTER JOIN?

How do LEFT JOIN, RIGHT JOIN, and FULL JOIN fit in?

Python join, why is it string.join(list) instead of list.join(string)?

This has always confused me. It seems like this would be nicer:

my_list = ["Hello", "world"]
print my_list.join("-")
# Produce: "Hello-world"

Than this:

my_list = ["Hello", "world"]
print "-".join(my_list)
# Produce: "Hello-world"

Is there a specific reason it does it like this?

Full Outer Join in MySQL

I want to do a Full Outer Join in MySQL. Is this possible? Is a Full Outer Join supported by MySQL?

What's the difference between INNER JOIN, LEFT JOIN, RIGHT JOIN and FULL JOIN? [duplicate]

What's the difference between INNER JOIN, LEFT JOIN, RIGHT JOIN and FULL JOIN in MySQL?

SQL update from one Table to another based on a ID match

I have a database with account numbers and card numbers. I match these to a file to update any card numbers to the account number, so that I am only working with account numbers.

I created a view linking the table to the account/card database to return the Table ID and the related account number, and now I need to update those records where the ID matches with the Account Number.

This is the Sales_Import table, where the account number field needs to be updated:

LeadID  AccountNumber
147         5807811235
150         5807811326
185         7006100100007267039

And this is the RetrieveAccountNumber table, where I need to update from:

LeadID  AccountNumber
147         7006100100007266957
150         7006100100007267039

I tried the below, but no luck so far:

UPDATE [Sales_Lead].[dbo].[Sales_Import] 
SET    [AccountNumber] = (SELECT RetrieveAccountNumber.AccountNumber 
                          FROM   RetrieveAccountNumber 
                          WHERE  [Sales_Lead].[dbo].[Sales_Import]. LeadID = 
                                                RetrieveAccountNumber.LeadID) 

It updates the card numbers to account numbers, but the account numbers gets replaced by NULL

INNER JOIN ON vs WHERE clause

For simplicity, assume all relevant fields are NOT NULL.

You can do:

SELECT
    table1.this, table2.that, table2.somethingelse
FROM
    table1, table2
WHERE
    table1.foreignkey = table2.primarykey
    AND (some other conditions)

Or else:

SELECT
    table1.this, table2.that, table2.somethingelse
FROM
    table1 INNER JOIN table2
    ON table1.foreignkey = table2.primarykey
WHERE
    (some other conditions)

Are those two worked on the same way by MySQL?

Difference between JOIN and INNER JOIN

Both these joins will give me the same results:

SELECT * FROM table JOIN otherTable ON table.ID = otherTable.FK

vs

SELECT * FROM table INNER JOIN otherTable ON table.ID = otherTable.FK

Is there any difference between the statements in performance or otherwise?

Does it differ between different SQL implementations?

Update a table using JOIN in SQL Server?

I want to update a column in a table making a join on other table e.g.:

UPDATE table1 a 
INNER JOIN table2 b ON a.commonfield = b.[common field] 
SET a.CalculatedColumn= b.[Calculated Column]
WHERE 
    b.[common field]= a.commonfield
AND a.BatchNO = '110'

But it is complaining :

Msg 170, Level 15, State 1, Line 2
Line 2: Incorrect syntax near 'a'.

What is wrong here?

How to join (merge) data frames (inner, outer, left, right)?

Given two data frames:

df1 = data.frame(CustomerId = c(1:6), Product = c(rep("Toaster", 3), rep("Radio", 3)))
df2 = data.frame(CustomerId = c(2, 4, 6), State = c(rep("Alabama", 2), rep("Ohio", 1)))

df1
#  CustomerId Product
#           1 Toaster
#           2 Toaster
#           3 Toaster
#           4   Radio
#           5   Radio
#           6   Radio

df2
#  CustomerId   State
#           2 Alabama
#           4 Alabama
#           6    Ohio

How can I do database style, i.e., sql style, joins? That is, how do I get:

• An inner join of df1 and df2:
  Return only the rows in which the left table have matching keys in the right table.
• An outer join of df1 and df2:
  Returns all rows from both tables, join records from the left which have matching keys in the right table.
• A left outer join (or simply left join) of df1 and df2
  Return all rows from the left table, and any rows with matching keys from the right table.
• A right outer join of df1 and df2
  Return all rows from the right table, and any rows with matching keys from the left table.

Extra credit:

How can I do a sql style select statement?

T-SQL: Selecting rows to delete via joins

Scenario:

Let's say I have two tables, TableA and TableB. TableB's primary key is a single column (BId), and is a foreign key column in TableA.

In my situation, I want to remove all rows in TableA that are linked with specific rows in TableB: Can I do that through joins? Delete all rows that are pulled in from the joins?

DELETE FROM TableA 
FROM
   TableA a
   INNER JOIN TableB b
      ON b.BId = a.BId
      AND [my filter condition]

Or am I forced to do this:

DELETE FROM TableA
WHERE
   BId IN (SELECT BId FROM TableB WHERE [my filter condition])

The reason I ask is it seems to me that the first option would be much more effecient when dealing with larger tables.

Thanks!

Rails :include vs. :joins

This is more of a "why do things work this way" question rather than a "I don't know how to do this" question...

So the gospel on pulling associated records that you know you're going to use is to use :include because you'll get a join and avoid a whole bunch of extra queries:

Post.all(:include => :comments)

However when you look at the logs, there's no join happening:

Post Load (3.7ms)   SELECT * FROM "posts"
Comment Load (0.2ms)   SELECT "comments.*" FROM "comments" 
                       WHERE ("comments".post_id IN (1,2,3,4)) 
                       ORDER BY created_at asc) 

It is taking a shortcut because it pulls all of the comments at once, but it's still not a join (which is what all the documentation seems to say). The only way I can get a join is to use :joins instead of :include:

Post.all(:joins => :comments)

And the logs show:

Post Load (6.0ms)  SELECT "posts".* FROM "posts" 
                   INNER JOIN "comments" ON "posts".id = "comments".post_id

Am I missing something? I have an app with half a dozen associations and on one screen I display data from all of them. Seems like it would be better to have one join-ed query instead of 6 individuals. I know that performance-wise it's not always better to do a join rather than individual queries (in fact if you're going by time spent, it looks like the two individual queries above are faster than the join), but after all the docs I've been reading I'm surprised to see :include not working as advertised.

Maybe Rails is cognizant of the performance issue and doesn't join except in certain cases?

When and why are database joins expensive?

I'm doing some research into databases and I'm looking at some limitations of relational DBs.

I'm getting that joins of large tables is very expensive, but I'm not completely sure why. What does the DBMS need to do to execute a join operation, where is the bottleneck?
How can denormalization help to overcome this expense? How do other optimization techniques (indexing, for example) help?

Personal experiences are welcome! If you're going to post links to resources, please avoid Wikipedia. I know where to find that already.

Thanks!

PS. In relation to this, I'm wondering about the denormalized approaches used by cloud service databases like BigTable and SimpleDB. See this question.

How do I perform the SQL Join equivalent in MongoDB?

How do I perform the SQL Join equivalent in MongoDB?

For example say you have two collections (users and comments) and I want to pull all the comments with pid=444 along with the user info for each.

comments
  { uid:12345, pid:444, comment="blah" }
  { uid:12345, pid:888, comment="asdf" }
  { uid:99999, pid:444, comment="qwer" }

users
  { uid:12345, name:"john" }
  { uid:99999, name:"mia"  }

Is there a way to pull all the comments with a certain field (eg. ...find({pid:444}) ) and the user information associated with each comment in one go?

At the moment, I am first getting the comments which match my criteria, then figuring out all the uid's in that result set, getting the user objects, and merging them with the comment's results. Seems like I am doing it wrong.