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Top sql frequently asked interview questions

When should I use Cross Apply over Inner Join?

What is the main purpose of using CROSS APPLY?

I have read (vaguely, through posts on the Internet) that cross apply can be more efficient when selecting over large data sets if you are partitioning. (Paging comes to mind)

I also know that CROSS APPLY doesn't require a UDF as the right-table.

In most INNER JOIN queries (one-to-many relationships), I could rewrite them to use CROSS APPLY, but they always give me equivalent execution plans.

Can anyone give me a good example of when CROSS APPLY makes a difference in those cases where INNER JOIN will work as well?


Edit:

Here's a trivial example, where the execution plans are exactly the same. (Show me one where they differ and where cross apply is faster/more efficient)

create table Company (
    companyId int identity(1,1)
,   companyName varchar(100)
,   zipcode varchar(10) 
,   constraint PK_Company primary key (companyId)
)
GO

create table Person (
    personId int identity(1,1)
,   personName varchar(100)
,   companyId int
,   constraint FK_Person_CompanyId foreign key (companyId) references dbo.Company(companyId)
,   constraint PK_Person primary key (personId)
)
GO

insert Company
select 'ABC Company', '19808' union
select 'XYZ Company', '08534' union
select '123 Company', '10016'


insert Person
select 'Alan', 1 union
select 'Bobby', 1 union
select 'Chris', 1 union
select 'Xavier', 2 union
select 'Yoshi', 2 union
select 'Zambrano', 2 union
select 'Player 1', 3 union
select 'Player 2', 3 union
select 'Player 3', 3 


/* using CROSS APPLY */
select *
from Person p
cross apply (
    select *
    from Company c
    where p.companyid = c.companyId
) Czip

/* the equivalent query using INNER JOIN */
select *
from Person p
inner join Company c on p.companyid = c.companyId

Source: (StackOverflow)

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


Source: (StackOverflow)

How do I limit the number of rows returned by an Oracle query after ordering?

Is there a way to make an Oracle query behave like it contains a MySQL limit clause?

In MySQL, I can do this:

select * 
from sometable
order by name
limit 20,10

to get the 21st to the 30th rows (skip the first 20, give the next 10). The rows are selected after the order by, so it really starts on the 20th name alphabetically.

In Oracle, the only thing people mention is the rownum pseudo-column, but it is evaluated before order by, which means this:

select * 
from sometable
where rownum <= 10
order by name

will return a random set of ten rows ordered by name, which is not usually what I want. It also doesn't allow for specifying an offset.


Source: (StackOverflow)

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?


Source: (StackOverflow)

Should I use != or <> for not equal in TSQL?

I have seen SQL that uses both != and <> for not equal. What is the preferred syntax and why?

I like != because <> reminds me of Visual Basic.


Source: (StackOverflow)

How to reset AUTO_INCREMENT in MySQL?

How can I reset the auto-increment of a field? I want it to start counting from 1 again.


Source: (StackOverflow)

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

This question already has an answer here:

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


Source: (StackOverflow)

Best way to get identity of inserted row?

What is the best way to get identity of inserted row?

I know about @@IDENTITY and IDENT_CURRENT and SCOPE_IDENTITY but don't understand the pros and cons attached to each.

Can someone please explain the differences and when I should be using each?


Source: (StackOverflow)

Can I concatenate multiple MySQL rows into one field?

Using MySQL, I can do something like:

SELECT hobbies FROM peoples_hobbies WHERE person_id = 5;

and get:

shopping
fishing
coding

but instead I just want 1 row, 1 col:

shopping, fishing, coding

The reason is that I'm selecting multiple values from multiple tables, and after all the joins I've got a lot more rows than I'd like.

I've looked for a function on MySQL Doc and it doesn't look like the CONCAT or CONCAT_WS functions accept result sets, so does anyone here know how to do this?


Source: (StackOverflow)

mysql_fetch_array() expects parameter 1 to be resource (or mysqli_result), boolean given

I am trying to select data from a MySQL table, but I get one of the following error messages:

mysql_fetch_array() expects parameter 1 to be resource, boolean given

or

mysqli_fetch_array() expects parameter 1 to be mysqli_result, boolean given

This is my code:

$username = $_POST['username'];
$password = $_POST['password'];

$result = mysql_query('SELECT * FROM Users WHERE UserName LIKE $username');

while($row = mysql_fetch_array($result)) {
    echo $row['FirstName'];
}

The same applies to code like

$result = mysqli_query($mysqli, 'SLECT ...');
// mysqli_fetch_array() expects parameter 1 to be mysqli_result, boolean given
while( $row=mysqli_fetch_array($result) ) {
    ...

and

$result = $mysqli->query($mysqli, 'SELCT ...');
// Call to a member function fetch_assoc() on a non-object
while( $row=$result->fetch_assoc($result) ) {
    ...

and

$result = $pdo->query('SLECT ...', PDO::FETCH_ASSOC);
// Invalid argument supplied for foreach()
foreach( $result as $row ) {
    ...

and

$stmt = $mysqli->prepare('SLECT ...');
// Call to a member function bind_param() on a non-object
$stmt->bind_param(...)

and

$stmt = $pdo->prepare('SLECT ...');
// Call to a member function bindParam() on a non-object
$stmt->bindParam(...)

Source: (StackOverflow)

Inserting multiple rows in a single SQL query? [duplicate]

I have multiple set of data to insert at once, say 4 rows. My table has three columns: Person, Id and Office.

INSERT INTO MyTable VALUES ("John", 123, "Lloyds Office");
INSERT INTO MyTable VALUES ("Jane", 124, "Lloyds Office");
INSERT INTO MyTable VALUES ("Billy", 125, "London Office");
INSERT INTO MyTable VALUES ("Miranda", 126, "Bristol Office");

Can I insert all 4 rows in a single SQL statement?


Source: (StackOverflow)

Left join and Left outer join in SQL Server

What is the difference between left join and left outer join?


Source: (StackOverflow)

How to perform an IF...THEN in an SQL SELECT?

How do I perform an IF...THEN in an SQL SELECT statement?

For example:

SELECT IF(Obsolete = 'N' OR InStock = 'Y' ? 1 : 0) AS Saleable, * FROM Product

Source: (StackOverflow)

How to list the tables in an SQLite database file that was opened with ATTACH?

What SQL can be used to list the tables, and the rows within those tables in a SQLite database file - once I have attached it with the ATTACH command on the SQLite 3 command line tool?


Source: (StackOverflow)

Table Naming Dilemma: Singular vs. Plural Names [closed]

Convention has it that table names should be the singular of the entity that they store attributes of.

I dislike any T-SQL that requires square brackets around names, but I have renamed a Users table to the singular, forever sentencing those using the table to sometimes have to use brackets.

My gut feel is that it is more correct to stay with the singular, but my gut feel is also that brackets indicate undesirables like column names with spaces in them etc.

Should I stay, or should I go?


Source: (StackOverflow)