Fastest Way to Determine If Record Exists

Fastest way to determine if record exists

SELECT TOP 1 products.id FROM products WHERE products.id = ?; will outperform all of your suggestions as it will terminate execution after it finds the first record.

SQL: How to properly check if a record exists

It's better to use either of the following:

-- Method 1.
SELECT 1
FROM table_name
WHERE unique_key = value;

-- Method 2.
SELECT COUNT(1)
FROM table_name
WHERE unique_key = value;

The first alternative should give you no result or one result, the second count should be zero or one.

How old is the documentation you're using? Although you've read good advice, most query optimizers in recent RDBMS's optimize SELECT COUNT(*) anyway, so while there is a difference in theory (and older databases), you shouldn't notice any difference in practice.

Fastest way to check the records if exists in the SQL table

First, You should use the EXISTS statement instead of selecting top 1:

SET @getRowCount = EXISTS(select 1
       from MYTable 
       where ID IN ('3','5','2','4','1') 
       AND datewithTime >= '2015-01-01 07:00:00' 
       AND datewithTime < '2016-01-01 07:00:00'
)

Second, you should check the execution plan to see if you can improve performance by adding indices or altering existing indices.

update

Sorry, I wasn't paying enough attention to what I'm writing.
Exists returns a boolean value, but sql server does not have a boolean data type, this is why you get the incorrect syntax error.

Here is the correct syntax:

DECLARE  @getRowCount bit = 0

IF EXISTS(select 1
       from MYTable 
       where ID IN ('3','5','2','4','1') 
       AND datewithTime >= '2015-01-01 07:00:00' 
       AND datewithTime < '2016-01-01 07:00:00'
) SET @getRowCount = 1

SELECT @getRowCount

How to check if record exists in database - fastest method

To ensure the fastest processing, make sure:

• The field you are searching on is indexed (you told about an "unique" string, so I suppose it is already the case. For this reason, "limit 1" is not necessary. Otherwise, it should be added)
• You are using the ExecuteScalar() method of your Command object

Cleanest way to check if a record exists

Q: Should I have a LIMIT 1 at the end of my SQL statement? Does COUNT(1) already limit the amount of rows found by 1 or does the server keep searching for more records even after finding the first one?

Your SELECT COUNT() FROM query will return one row, if the execution is successful, because there is no GROUP BY clause. There's no need to add a LIMIT 1 clause, it wouldn't have any affect.

The database will search for all rows that satisfy the conditions in the WHERE clause. If the user_id column is UNIQUE, and there is an index with that as the leading column, or, if that column is the PRIMARY KEY of the table... then the search for all matching rows will be efficient, using the index. If there isn't an index, then MySQL will need to search all the rows in the table.

It's the index that buys you good performance. You could write the query differently, to get a usable result. But what you have is fine.

Q: Is this the cleanest...

  if ($stmt->fetch()[0]>0)

My personal preference would be to avoid that construct, and break that up into two or more statements. The normal pattern...separate statement to fetch the row, and then do a test.

Personally, I would tend to avoid the COUNT() and just get a row, and test whether there was a row to fetch...

  $sql = "SELECT 1 AS `row_exists` FROM myTable WHERE user_id = :id_var";
  $stmt = $conn->prepare($sql);
  $stmt->bindParam(':id_var', $id_var);
  $stmt->execute();
  if($stmt->fetch())  {
      // row found
  } else {
      // row not found 
  }
  $stmt->closeCursor(); 

What is the best way to check if a record exists in a SQL Server table using C#?

Exists is more efficient than Count, because count needs to scan all rows to match the criteria and include in the count, exist dont.

So exists with ExecuteScalar is better.

As more info backing this:

According to http://sqlblog.com/blogs/andrew_kelly/archive/2007/12/15/exists-vs-count-the-battle-never-ends.aspx

Both queries scanned the table but the EXISTS was able to at least do
a partial scan do to the fact it can stop after it finds the very
first matching row. Where as the COUNT() must read each and every row
in the entire table to determine if they match the criteria and how
many there are. That is the key folks. The ability to stop working
after the first row that meets the criteria of the WHERE clause is
what makes EXISTS so efficient. The optimizer knows of this behavior
and can factor that in as well. Now keep in mind that these tables are
relatively small compared to most databases in the real world. So the
figures of the COUNT() queries would be multiplied many times on
larger tables. You could easily get hundred's of thousands of reads or
more on tables with millions of rows but the EXISTS will still only
have just a few reads on any queries that can use an index to satisfy
the WHERE clause.

As a simple experiment using AdventureWorks with MSSQL 2012

set showplan_all on

-- TotalSubtreeCost: 0.06216168
select count(*) from sales.Customer

-- TotalSubtreeCost: 0.003288537
select 1 where exists (select * from sales.Customer)

See also

http://sqlmag.com/t-sql/exists-vs-count

UPDATE: On ExecuteScalar vs ExecuteReader.
Having a look with a disassembler (like Reflector) on the Implementation of System.Data.SqlClient.SqlCommand methods, shows something surprising, they are kind of equivalent: both end up calling the internal helper
internal SqlDataReader RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, bool returnStream, string method, TaskCompletionSource completion, int timeout, out Task task, bool asyncWrite = false)

which returns a SqlDataReader, the ExecuteReader returns it as is.
While ExecuteScalar consumes it with another helper:

private object CompleteExecuteScalar(SqlDataReader ds, bool returnSqlValue)
{
    object obj2 = null;
    try
    {
        if (!ds.Read() || (ds.FieldCount <= 0))
        {
            return obj2;
        }
        if (returnSqlValue)
        {
            return ds.GetSqlValue(0);
        }
        obj2 = ds.GetValue(0);
    }
    finally
    {
        ds.Close();
    }
    return obj2;
}

As a side note, same goes with MySQL Connector/NET (The official ADO.NET open source driver for MySQL), the method ExecuteScalar internally creates an DataReader (MySqlDataReader to be more precise) and consumes it. See on source file /Src/Command.cs (from https://dev.mysql.com/downloads/connector/net/ or https://github.com/mysql/mysql-connector-net).

Summary: Regarding the ExecuteScalar vs ExecuteReader both incurr in the overhead of creating a SqlDataReader, I would say the difference is mostly idiomatic.

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