How do I select time only from a DATETIME column?

You have two options that come to mind straight away. 
 
One is to use the CONVERT function in conjunction with specific style numbers, to convert the date value into a specific type of string. I like using 114, because you can simply change the number of characters returned to increase accuracy. For example, for HH:MM: 
 

SELECT      CONVERT(CHAR(5), DateColumn, 114)  FROM      table  [WHERE ... ]

 
Will produce the following: 
 

11:45

 
For HH:MM:SS: 
 

SELECT      CONVERT(CHAR(8), DateColumn, 114)  FROM      table  [WHERE ... ]

 
Will produce the following: 
 

11:45:37

 
And for HH:MM:SS:mmm: 
 

SELECT      CONVERT(CHAR(12), DateColumn, 114)  FROM      table  [WHERE ... ]

 
Will produce the following: 
 

11:45:37:623

 
Depending on your application, you may need any of the above accuracies. Usually, though, to-the-minute is sufficient. 
 

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The other option is to use DATEPART to concatenate the time yourself. This is a bit messier, but is useful, for example, if you only want the minutes and are not concerned about the actual hour. I can't think of a practical use for this, but one must exist. 
 

SELECT      DATEPART(MINUTE, DateColumn)  FROM      table  [WHERE ... ]

 
If you want to build an entire time string on your own, you could do this: 
 

SELECT      CONVERT(VARCHAR(2),DATEPART(HOUR, DateColumn)) + ':' +      CONVERT(VARCHAR(2),DATEPART(MINUTE, DateColumn))  FROM      table  [WHERE ... ]

 
Now, you might find that minutes less than 10 will result in weird padding; for example, if you were going to use this technique for building a time string on your own, you could end up with a result as follows: 
 

5:3

 
This would be 5:03, but could obviously be misconstrued by the user. Here is how I work around this scenario: 
 

SELECT      CONVERT(VARCHAR(2), DATEPART(HOUR, DateColumn)) + ':' +      CASE          WHEN DATEPART(MINUTE, DateColumn) < 10 THEN              '0'+CONVERT(VARCHAR(2),DATEPART(MINUTE, DateColumn))          ELSE              CONVERT(VARCHAR(2),DATEPART(MINUTE, DateColumn))      END  FROM      table  [WHERE ...]