Calculate AVERAGE from 2 columns for each row in SQL

问题

As an example I will have this table from MySQL

Id  | Name  | Grade 1  | Grade 2 | Average

1.  | Jack  | 9        | 10      |
2.  | Jimmy | 9        | 8       |
2.  | Emmy  | 9        | 7       |

So, in the Average field from this table, I need to calculate the AVERAGE from the Grade 1 and Grade 2 fields. I tried a lot of possiblities which I know they are wrong like:

UPDATE table_name SET Average=AVG(Grade 1 + Grade 2)

I there a way to do this? Can anyone help me? Thanks!

回答1:

You need to add the fields together and divide by the number of fields. If your Average field is of DECIMAL type you don't really even need to specify the ROUND function. Any decimal exceeding the declaration will just be truncated (SQL Fiddle) :

UPDATE table_name 
SET AVERAGE = (grade1 + grade2) / 2;

In your example you only have two fields that you are getting the average of. So Average decimal(3,1) would work for you since the most the decimal portion will ever be is .5. So the ROUND function is clearly not needed.

来源：https://stackoverflow.com/questions/26280947/calculate-average-from-2-columns-for-each-row-in-sql