$add with some fields as Null returning sum value as Null

问题

These are the documents in my collection

{ "_id" : 1, "quizzes" : [ 10, 6, 7 ], "labs" : [ 5, 8 ], "final" : 80, "midterm" : 75 ,"extraMarks":10}
{ "_id" : 2, "quizzes" : [ 9, 10 ], "labs" : [ 8, 8 ], "final" : 95, "midterm" : 80 }
{ "_id" : 3, "quizzes" : [ 4, 5, 5 ], "labs" : [ 6, 5 ], "final" : 78, "midterm" : 70 }

Only Document 1 has a field extra marks:

Now i have to make a projection as a sum of "final"+"midterm"+"extraMarks"

I have written a query for projection as follows:

db.students.aggregate([    {      $project: {          examTotal: { $add: [ "$final", "$midterm","$extraMarks" ] }      }    } ])

This query is giving me correct result for Document1 and for Doc2 and Doc3, as the field doesnt exist its giving sum as null.

Is it possible to check if the field is not null and add in the query itself..Any suggestions for this?

Is there any functionality similar to nvl (in SQL) in the queries Mongo DB?

回答1:

You can do two projections, the first using $ifNull which is similar to nvl:

db.students.aggregate([
  { $project: { final: 1, midterm: 1, extraMarks: { $ifNull: [ "$extraMarks", 0 ] } } }, 
  { $project: { examTotal: { $add: [ "$final", "$midterm","$extraMarks" ] } } } 
])

回答2:

Use the $match stage, add it to the beginning of your pipeline {$match: {"your_field":{$exists: true}}}

来源：https://stackoverflow.com/questions/35250675/add-with-some-fields-as-null-returning-sum-value-as-null