How do you test an element for existence without the use of the `getElementById` method?

I have set up a [live demo][1] for reference. I will also print the code on here as well:

    &lt;!DOCTYPE html&gt;
    &lt;html&gt;
    &lt;head&gt;
        &lt;script&gt;
        var getRandomID = function (size) {
                var str = &quot;&quot;,
                    i = 0,
                    chars = &quot;0123456789abcdefghijklmnopqurstuvwxyzABCDEFGHIJKLMNOPQURSTUVWXYZ&quot;;
                while (i &lt; size) {
                    str += chars.substr(Math.floor(Math.random() * 62), 1);
                    i++;
                }
                return str;
            },
            isNull = function (element) {
                var randomID = getRandomID(12),
                    savedID = (element.id)? element.id : null;
                element.id = randomID;
                var foundElm = document.getElementById(randomID);
                element.removeAttribute(&#39;id&#39;);
                if (savedID !== null) {
                    element.id = savedID;
                }
                return (foundElm) ? false : true;
            };
        window.onload = function () {
            var image = document.getElementById(&quot;demo&quot;);
            console.log(&#39;undefined&#39;, (typeof image === &#39;undefined&#39;) ? true : false); // false
            console.log(&#39;null&#39;, (image === null) ? true : false); // false
            console.log(&#39;find-by-id&#39;, isNull(image)); // false
            image.parentNode.removeChild(image);
            console.log(&#39;undefined&#39;, (typeof image === &#39;undefined&#39;) ? true : false); // false ~ should be true?
            console.log(&#39;null&#39;, (image === null) ? true : false); // false ~ should be true?
            console.log(&#39;find-by-id&#39;, isNull(image)); // true ~ correct but there must be a better way than this?
        };
        &lt;/script&gt;
    &lt;/head&gt;
    &lt;body&gt;
        &lt;div id=&quot;demo&quot;&gt;&lt;/div&gt;
    &lt;/body&gt;
    &lt;/html&gt;

Basically the above code demonstrates an element being stored into a variable and then removed from the DOM. Even though the element has been removed from the DOM, the variable retains the element as it was when first declared. In other words, it is not a live reference to the element itself, but rather a replica. As a result, checking the variable&#39;s value (the element) for existence will provide an unexpected result.

The `isNull` function is my attempt to check for an elements existence from a variable, and it works, but I would like to know if there is an easier way to accomplish the same result.

PS: I&#39;m also interested in why JavaScript variables behave like this if anyone knows of some good articles related to the subject.

  [1]: http://jsbin.com/apawi5/3





How can I check if an element exists in the visible DOM?

How can I check if any of the strings in an array exists in another string?

Like:

    a = [&#39;a&#39;, &#39;b&#39;, &#39;c&#39;]
    str = &quot;a123&quot;
    if a in str:
      print &quot;some of the strings found in str&quot;
    else:
      print &quot;no strings found in str&quot;

That code doesn&#39;t work, it&#39;s just to show what I want to achieve.

Check if multiple strings exist in another string

What is the difference between the `EXISTS` and `IN` clause in SQL?

When should we use `EXISTS`, and when should we use `IN`?


Difference between EXISTS and IN in SQL?

I&#39;m trying to find out if a row exists in a table. Using MySQL, is it better to do a query like this:

    SELECT COUNT(*) AS total FROM table1 WHERE ...

and check to see if the total is non-zero or is it better to do a query like this:

    SELECT * FROM table1 WHERE ... LIMIT 1

and check to see if any rows were returned?

In both queries, the WHERE clause uses an index.

Best way to test if a row exists in a MySQL table

Is it possible to check if a (MySQL) database exists after having made a connection.

I know how to check if a table exists in a DB, but I need to check if the DB exists. If not I have to call another piece of code to create it and populate it.

I know this all sounds somewhat inelegant - this is a quick and dirty app.

How to check if mysql database exists

Yes, I know this has been asked a lot.
But, it confuses me, since the results on google for this search show different methods (listed below)

    $(document).ready(function() {
        if ($(&#39;#DivID&#39;).length){
            alert(&#39;Found with Length&#39;);
        }
        
        if ($(&#39;#DivID&#39;).length &gt; 0 ) {
            alert(&#39;Found with Length bigger then Zero&#39;);
        }
        
        if ($(&#39;#DivID&#39;) != null ) {
            alert(&#39;Found with Not Null&#39;);
        }
    });

Which one of the 3 is the correct way to check if the div exists?

EDIT:
It&#39;s a pitty to see that people do not want to learn what is the better approach from the three different methods. This question is not actually on &quot;How to check if a div exists&quot; but it&#39;s about which method is better, and, if someone could explain, why it it better?

Check if a div exists with jquery

I&#39;ve found a few &quot;would be&quot; solutions for the classic &quot;How do I insert a new record or update one if it already exists&quot; but I cannot get any of them to work in SQLite.

I have a table defined as follows:

    CREATE TABLE Book 
    ID     INTEGER PRIMARY KEY AUTOINCREMENT,
    Name   VARCHAR(60) UNIQUE,
    TypeID INTEGER,
    Level  INTEGER,
    Seen   INTEGER

What I want to do is add a record with a unique Name. If the Name already exists, I want to modify the fields. 

Can somebody tell me how to do this please?



INSERT IF NOT EXISTS ELSE UPDATE?

I changed the datadir of a MySQL installation and all the bases moved correctly except for one. 
I can connect and `USE` the database. `SHOW TABLES` also returns me all the tables correctly, and the files of each table exists on the MySQL data directory.

However, when I try to `SELECT` something from the table, I get an error message that the table does not exist. Yet, this does not make sense since I was able to show the same table through `SHOW TABLES` statement. 

My guess is that `SHOW TABLES` lists file existence but does not check whether a file is corrupted or not. Consequently, I can list those files but not access them.

Nevertheless, it is merely a guess. I have never seen this before. Now, I cannot restart the database for testing, but every other application that uses it is running fine.
But that&#39;s just a guess, I&#39;ve never seen this before. 

Does anyone know why this is happening?

Example:

    mysql&gt; SHOW TABLES;
    +-----------------------+
    | Tables_in_database    |
    +-----------------------+
    | TABLE_ONE             |
    | TABLE_TWO             |
    | TABLE_THREE           |
    +-----------------------+
    mysql&gt; SELECT * FROM TABLE_ONE;
    ERROR 1146 (42S02): Table &#39;database.TABLE_ONE&#39; doesn&#39;t exist



MySQL &gt; Table doesn&#39;t exist. But it does (or it should)

I&#39;ve got two postgresql tables:  

&lt;!-- language: none --&gt;

    table name     column names
    -----------    ------------------------
    login_log      ip | etc.
    ip_location    ip | location | hostname | etc.

I want to get every IP address from `login_log` which doesn&#39;t have a row in `ip_location`.  
I tried this query but it throws a syntax error.

    SELECT login_log.ip 
    FROM login_log 
    WHERE NOT EXIST (SELECT ip_location.ip
                     FROM ip_location
                     WHERE login_log.ip = ip_location.ip)

&gt;     ERROR: syntax error at or near &quot;SELECT&quot;
&gt;     LINE 3: WHERE NOT EXIST (SELECT ip_location.ip`

I&#39;m also wondering if this query (with adjustments to make it work) is the best performing query for this purpose.


Select rows which are not present in other table

My query is as follows, and contains a subquery within it:

     select count(distinct dNum)
     from myDB.dbo.AQ
     where A_ID in 
      (SELECT DISTINCT TOP (0.1) PERCENT A_ID, 
    			COUNT(DISTINCT dNum) AS ud 
     FROM         myDB.dbo.AQ
     WHERE     M &gt; 1 and B = 0 
     GROUP BY A_ID ORDER BY ud DESC)


The error I am receiving is ...

    Only one expression can be specified in the select list when the subquery is not
    introduced with EXISTS.`

When I run the sub-query alone, it returns just fine, so I am assuming there is some issue with the main query?

Only one expression can be specified in the select list when the subquery is not introduced with EXISTS

I have a list of `orders`.  
I want to select `orders` based on a set of order statuses.

So essentially `select orders where order.StatusCode in (&quot;A&quot;, &quot;B&quot;, &quot;C&quot;)`

    // Filter the orders based on the order status
    var filteredOrders = from order in orders.Order
                         where order.StatusCode.????????(&quot;A&quot;, &quot;B&quot;, &quot;C&quot;)
                         select order;

Linq select objects in list where exists IN (A,B,C)

I&#39;m curious which of the following below would be more efficient?

I&#39;ve always been a bit cautious about using `IN` because I believe SQL Server turns the result set into a big `IF` statement. For a large result set, this could result in poor performance. For small result sets, I&#39;m not sure either is preferable. For large result sets, wouldn&#39;t `EXISTS` be more efficient?

    WHERE EXISTS (SELECT * FROM Base WHERE bx.BoxID = Base.BoxID AND [Rank] = 2)

vs.

    WHERE bx.BoxID IN (SELECT BoxID FROM Base WHERE [Rank = 2])

SQL Server IN vs. EXISTS Performance

I need to check if a file is on HDD at a specified location ($path.$file_name).   

Which is the difference between [`is_file()`][1] and [`file_exists()`][2] functions and which is better/faster to use in PHP?


  [1]: http://www.php.net/manual/en/function.is-file.php
  [2]: http://www.php.net/manual/en/function.file-exists.php

is_file or file_exists in PHP

I have an `Email` document which has a `sent_at` date field:

    {
      &#39;sent_at&#39;: Date( 1336776254000 )
    }

If this `Email` has not been sent, the `sent_at` field is either null, or non-existant.

I need to get the count of all sent/unsent `Emails`. I&#39;m stuck at trying to figure out the right way to query for this information. I think this is the right way to get the sent count:

    db.emails.count({sent_at: {$ne: null}})

But how should I get the count of the ones that aren&#39;t sent?

MongoDB: How to query for records where field is null or not set?

How to search for documents in a collection that are missing a certain field in MongoDB?

Mongo: find items that don&#39;t have a certain field

What is the best way to check if an object exists in the database from a performance point of view? I&#39;m using Entity Framework 1.0 (ASP.NET 3.5 SP1).


Best way to check if object exists in Entity Framework?

I need to generate a directory in my makefile and I would like to not get the &quot;directory already exists error&quot; over and over even though I can easily ignore it.

I mainly use mingw/msys but would like something that works across other shells/systems  too.

I tried this but it didn&#39;t work, any ideas?

    ifeq (,$(findstring $(OBJDIR),$(wildcard $(OBJDIR) )))
	-mkdir $(OBJDIR)
    endif



how to prevent &quot;directory already exists error&quot; in a makefile when using mkdir

In my app a User can create a Business. When they trigger the `index` action in my `BusinessesController` I want to check if a Business is related to the `current_user.id`:

- If yes: display the business. 
- If no: redirect to the ` new ` action. 

I was trying to use this:

    if Business.where(:user_id =&gt; current_user.id) == nil
      # no business found
    end

But it always returns true even when the business doesn&#39;t exist...

**How can I test if a record exists in my database?**

Check if record exists from controller in Rails

I have a definition table that I know is not being maintained very well, lets call this `table A`. I have another table (call it `table B`) that is much smaller and ideally should be a `subset of table A` but I know that `table A` is somewhat stale and does not contain new entries that are in `Table B`.

Note, that tables A and B have different columns.

    Table A:
    ID, Name, blah, blah, blah, blah
    
    Table B:
    ID, Name


I want all rows in Table B such that the ID in Table B does NOT exist in Table A. This does not just match rows in Table A; I want only rows in Table B where the ID does NOT exist at all in Table A.

sql: check if entry in table A exists in table B

What I&#39;m trying to do is `INSERT` subscribers in my database, but `IF EXISTS` it should `UPDATE` the row, `ELSE INSERT INTO` a new row.

Ofcourse I connect to the database first and `GET` the `$name`, `$email` and `$birthday` from the url string.

    $con=mysqli_connect(&quot;localhost&quot;,&quot;---&quot;,&quot;---&quot;,&quot;---&quot;);
    // Check connection
    if (mysqli_connect_errno())
      {
      echo &quot;Failed to connect to MySQL: &quot; . mysqli_connect_error();
      }
    
    $name=$_GET[&#39;name&#39;]; 
    $email=$_GET[&#39;email&#39;];
    $birthday=$_GET[&#39;birthday&#39;];

This works, but just adds the new row;

    mysqli_query($con,&quot;INSERT INTO subs (subs_name, subs_email, subs_birthday)
    VALUES (&#39;$name&#39;, &#39;$email&#39;, &#39;$birthday&#39;)&quot;);
        
    mysqli_close($con);

Here&#39;s what I tried;

    mysqli_query($con,&quot;INSERT INTO subs (subs_name, subs_email, subs_birthday)
    VALUES &#39;$name&#39;, &#39;$email&#39;, &#39;$birthday&#39;
    ON DUPLICATE KEY UPDATE subs_name = VALUES($name), subs_birthday = VALUES($birthday)&quot;);
    mysqli_close($con);

and

    mysqli_query($con,&quot;IF EXISTS (SELECT * FROM subs WHERE subs_email=&#39;$email&#39;)
        UPDATE subs SET subs_name=&#39;$name&#39;, subs_birthday=&#39;$birthday&#39; WHERE subs_email=&#39;$email&#39;
    ELSE
        INSERT INTO subs (subs_name, subs_email, subs_birthday) VALUES (&#39;$name&#39;, &#39;$email&#39;, &#39;$birthday&#39;)&quot;);
    mysqli_close($con);

and

    mysqli_query($con,&quot;IF NOT EXISTS(SELECT * FROM subs WHERE subs_email=&#39;$email&#39;)
    Begin
    INSERT INTO subs (subs_name, subs_email, subs_birthday)
    VALUES (&#39;$name&#39;, &#39;$email&#39;, &#39;$birthday&#39;)
    End&quot;);
    mysqli_close($con);

But none of them work, what am I doing wrong?

Any help is greatly appreciated!



SQL - IF EXISTS UPDATE ELSE INSERT INTO

How to select Boolean value from sub query with IF EXISTS statement (SQL Server)?

It should be something like :

    SELECT 
      TABEL1.Id, 
      NewFiled = (IF EXISTS(SELECT Id FROM TABLE2 WHERE TABLE2.ID = TABEL1.ID) 
                     SELECT &#39;TRUE&#39; 
                  ELSE 
                     SELECT &#39;FALSE&#39;) 
    FROM TABEL1

How to use SQL Select statement with IF EXISTS sub query?

Is there a single way of detecting if a directory/file/symlink/etc. entity (more generalized) exists? 

I need a single function because I need to check an array of paths that could be directories, files or symlinks. I know `File.exists?&quot;file_path&quot;` works for directories and files but not for symlinks (which is `File.symlink?&quot;symlink_path&quot;`). 


How to check if a directory/file/symlink exists with one command in Ruby

How do I check whether data in a query exists?

For example:

    users_query = User.query.filter_by(email=&#39;x@x.com&#39;)

How I can check whether users with that email exist?

I can check this with 

    users_query.count()

but how to check it with **exists**?

sqlalchemy exists for query

I have a tableA:
 

    ID value
     1  100
     2  101
     2  444
     3  501

Also TableB 
  

    ID Code
    1
    2

Now I want to populate col = code of table B if there exists ID = 2 in tableA. for multiple values , get max value.
else populate it with &#39;123&#39;. Now here is what I used:

    if exists (select MAX(value) from #A where id = 2)
     BEGIN
     update #B
     set code = (select MAX(value) from #A where id = 2)
     from #A
     END
    
     ELSE 
     
     update #B
     set code = 123
     from #B

I am sure there is some problem in BEGIN;END or in IF EXIST;ELSE.
Basically I want to by-pass the else part if select statement in IF-part exist and vice- versa. For example if select statement of IF=part is:
 

    (select MAX(value) from #A where id = 4)

 It should just populate 123, coz ID = 4 do not exist ! 

SQL Server: IF EXISTS ; ELSE

I&#39;ve got a fstream my_file(&quot;test.txt&quot;), but I don&#39;t know if test.txt exists. In case it exists, I would like to know if I can read it, too. How to do that?

*I use Linux.*

How to check if a file exists and is readable in C++?

I&#39;m writing a function in PL/pgSQL, and I&#39;m looking for the simplest way to check if a row exists.  
Right now I&#39;m SELECTing an `integer` into a `boolean`, which doesn&#39;t really work. I&#39;m not experienced with PL/pgSQL enough yet to know the best way of doing this.

Here&#39;s part of my function:

    DECLARE person_exists boolean;
    BEGIN

    person_exists := FALSE;

    SELECT &quot;person_id&quot; INTO person_exists
      FROM &quot;people&quot; p
    WHERE p.person_id = my_person_id
    LIMIT 1;

    IF person_exists THEN
      -- Do something
    END IF;

    END; $$ LANGUAGE plpgsql;

*Update* - I&#39;m doing something like this for now:

    DECLARE person_exists integer;
    BEGIN

    person_exists := 0;

    SELECT count(&quot;person_id&quot;) INTO person_exists
      FROM &quot;people&quot; p
    WHERE p.person_id = my_person_id
    LIMIT 1;

    IF person_exists &lt; 1 THEN
      -- Do something
    END IF;

PL/pgSQL checking if a row exists

I am not able to find a solution to my problem online.

I would like to call a function in Unix, pass in the path of a directory, and know if it exists. `opendir()` returns an error if a directory does not exist, but my goal is not to actually open, check the error, close it if no error, but rather just check if a file is a directory or not.

Is there any convenient way to do that please?


Checking if a directory exists in Unix (system call)

I&#39;m wondering if I can select the value of a column if the column exists and just select null otherwise.  In other words I&#39;d like to &quot;lift&quot; the select statement to handle the case when the column doesn&#39;t exist.

    SELECT uniqueId
        ,  columnTwo
        ,  /*WHEN columnThree exists THEN columnThree ELSE NULL END*/ AS columnThree
    FROM (subQuery) s

Note, I&#39;m in the middle to solidifying my data model and design.  I hope to exclude this logic in the coming weeks, but I&#39;d really like to move beyond this problem right because the data model fix is a more time consuming endeavor than I&#39;d like to tackle now.

Also note, I&#39;d like to be able to do this in one query.  So I&#39;m not looking for an answer like 

&gt; check what columns are on your sub query first.  Then modify your
&gt; query to appropriately handle the columns on your sub query.

Select columnValue if the column exists otherwise null

I currently do this to check if one of two elements exists:

    if ($(&quot;.element1&quot;).length &gt; 0 || $(&quot;.element2&quot;).length &gt; 0) {
      //do stuff...
    }

Is there a better way to rewrite the same? I mean, is `.length` the same as `.length &gt; 0`?

If one of two elements exists, do something

How do you check if an include / require_once exists before you call it, I tried putting it in an error block, but PHP didn&#39;t like that.

I think `file_exists()` would work with some effort, however that would require the whole file path, and a relative include could not be passed into it easily.

Are there any other ways?

Check if an include (or require) exists

If we have a `SELECT` statement inside an `IF EXISTS`, does the execution stop as soon as it finds a record in the table? For example:


    IF EXISTS(SELECT *  FROM  table1  WHERE Name=&#39;John&#39; )
    
    return 1
    
    else
    
    return 0

If a row exists in the table with the name = John, does it stops execution and returns 1 or does it traverses through the entire table looking for more matches?

 

All Exists Solutions on Gang of Coders

Total of 31 Exists Solutions