Single criteria conditional formula is used when we need to add range of cells or values provides it meets the given criteria.  The SUMIF is the formula useful when we need to compute conditional addition. Simple example to use conditional sum is to calculate total marks obtained by only boys in a class room. Let us first look into the syntax of SUMIF function.
Syntax of SUMIF Function:
SUMIF(range, criteria, [sum_range])
Where 
range:The range of cells that you want evaluated by criteria. Cells in each range must be numbers or names, arrays, or references that contain numbers. Blank and text values are ignored.
criteria:The criteria in the form of a number, expression, a cell reference, text, or a function that defines which cells will be added. For example, criteria can be expressed as 32, ">32", B5, 32, "32", "apples", or TODAY(). 
sum_range: Optional. The actual cells to add, if you want to add cells other than those specified in the range argument. If the sum_range argument is omitted, Excel adds the cells that are specified in the range argument (the same cells to which the criteria is applied).
Note:Any text criteria or any criteria that includes logical or mathematical symbols must be enclosed in double quotation marks ("). If the criteria is numeric, double quotation marks are not required.

Examples for SUMIF function of Excel:

The above example describes a simple data of a mobile store which buys and sells mobile phones.  Let us use sumif function to get sum of data based on different criteria mentioned below.
1. Summing only negative values
The following formula is used to return the sum of the negative values in cell E8. In other words, it returns only the sum of negetive values( Only phones got sold in loss)
=SUMIF(E3:E7," Less than 0") 
(Use "<" symbol instead of  'Less than' in this formula. Due to technical limitations i am unable use that symbol directly in the formula above)
The SUMIF function can use three arguments. However if we omit the third argument, the second argument applies to the values in the range we have selected. That is E2:E6.
2. Summing  values based on a text comparison:
The following formula returns the sum of the bought price of only Xperia S smartphone.
"SUMIF(B2:B7,"Xperia S",C2:C7)"
3. Summing values based on a different range:
The following formula returns the sum of bought price where store made loss.
=SUMIF(E2:E7," Less than 0",C2:C7)
(Use "<" symbol instead of "Less than" in this formula. Due to technical limitations i am unable use that symbol directly in the formula above)
4. Summing values based on a date comparison:
The following formula returns the total loss in the before end of September month end.
SUMIF(A2:A7,"<"&DATE(2012,9,30),C2:C7)
Here the second argument for the SUMIF function is an expression. The expression uses the DATE function, which returns date. Also, the comparison operator enclosed in quotation marks is concatenated(Using the & operator) with the result of the DATE function. 

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