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Esteban Baez
Asked: 2020-05-10 20:23:53 +0800 CST 2020-05-10 20:23:53 +0800 CST

Query of approved subjects does not return expected result

  • 772

I have a problem with this query:

db.estudiantes.find({materias:{$gt:"3.0"}})

The intention of this is to find students who have failed at least one subject (the minimum passing grade is 3.0)

The data has this structure:

db.estudiantes.insert({
  id:"1000",
  nombre:"Diego Agreda",
  edad:"15",
  programa:"informatica",
  materias:[{programacion:"5.0",leyes:"2.1",ingles:"4.1"}]
})

When launching the query

 db.estudiantes.find({materias:{$gt:"3.0"}})

It doesn't give me any results. Why does that happen? How can I solve that?

mongodb

3 Answers

  1. Best Answer

    I share with you the mongo operators, so you can analyze which option you are looking for with your query.

    $lt     menor que           <

$lte    menor igual que     <=

$gt     mayor que           >

$gte    mayor igual que     >=

$ne     no igual            ¡=

    Other points to consider are:

    • First the field is _id to save it.
    • Your fields must be numeric, to be able to do the comparison with the operators gte, lte, gt, lt, etc.
    • You must access the field, since it is an embedded field, you must do it as follows 'as shown in the query'
    • The operator is $gte since you tell it to be greater than or equal to, otherwise it will simply be greater than.
    • The {name:1} refers to the field that I want to display

    Replicating your insert, with modifications that I consider you should make.

    db.estudiantes.insert({_id:1000,nombre:"Diego Agreda",
    edad:15,programa:"informatica",materias:[{programacion:5.0,leyes:2.1,ingles:4.1}]})

    Now the query:

        db.estudiantes.find({'materias.programacion':{$gte:3}}, {nombre:1})

    From what you tell me, from {name:1}, it is the visibility of the fields that I want to show, it is the mysql equivalent of:

    select nombre from estudiantes;

    Not every time we want to show all the fields of our tables, in this case collection.

    Example:

    db.e5.find({vistas:{$gte:400}},{github:1,titulo:1,tiempoblog:1})

    Then:

    • We place between {}, the name of the field followed by ':' and with the 1 we indicate true, that is to say that we want the result to be visible is the following.
    • Take an example that I already had, fictitious data.

    Result:

    enter image description here

    • 5

  2. Thanks to the contribution of annelyzzye and angel, I solved the scheme and the query, which turned out like this

    db.estudiantes.insert({id:1,nombre:"Esteban Baez",edad:20,programa:"informatica",materias:[{nombre:"programacion",calificacion:5.0},{nombre:"leyes",calificacion:5.0},{nombre:"ingles",calificacion:5.0}]})

    and the query

    db.estudiantes.find({'materias.calificacion':{$lt:3.0}})
    • 4

  3. If you modify your schema, in the matter property like this:

    materias:[{ nombre: 'Programacion', calificacion: '3.0'}]

    You can filter more easily in the following way:

     db.estudiantes.find({'materias.calificacion':{$lt:3}}, {nombre:1})

    In this way you get all the students who have subjects with grades lower than 3.0, since according to what you mention it 3.0is passing.

    Explanation

    This way of working is that a user can have an infinite number of subjects. A fix will allow us to have these subjects dynamically and filter the results more easily (in future queries).

    The code is on the fly. It may have errors.

    • 3