# Commands

By using the where method provided by the database API, we can construct complex query conditions to perform complex query tasks. In this chapter, we continue to use the sample data for the previous chapter.

# Query Commands

Assume we need to query the to-dos whose progress is greater than 30%. In this case, we cannot use congruent matching with the incoming object expression. Therefore, we need a query command. The database API provides multiple query commands, such as "greater than" and "less than", which are all exposed on the db.command object. For example, to query the todos whose progress is greater than 30%:

const _ = db.command
db.collection('todos').where({
  // The gt method is used to specify a "greater than" condition, where _.gt(30) is a "greater than 30" condition
  progress: _.gt(30)
})
.get({
  success: function(res) {
    console.log(res.data)
  }
})

The API provides the following query commands:

For details, refer to the Database APIs documentation.

# Logic Commands

In addition to specifying a condition that a field must satisfy, we can also query the todos whose progress is between 30% and 70% by specifying multiple conditions that a field must satisfy, for example, using an and logic command:

const _ = db.command
db.collection('todos').where({
  // The and method is used to specify an "and" condition, which means the two conditions, _.gt(30) and _.lt(70), must both be satisfied.
  progress: _.gt(30).and(_.lt(70))
})
.get({
  success: function(res) {
    console.log(res.data)
  }
})

Since there is an and, there should also be an or that we can use to query the todos whose progress is 0 or 100:

const _ = db.command
db.collection('todos').where({
  // The or method is used to specify an "or" condition, which means either of the conditions, _.eq(0) or _.eq(100), must be satisfied.
  progress: _.eq(0).or(_.eq(100))
})
.get({
  success: function(res) {
    console.log(res.data)
  }
})

We can use the "or" operation across fields. The or command can also be used for multiple (two or more) query conditions, indicating that one of the multiple query conditions must be met. For example, to query the todos whose progress is less than or equal to 50% with a white or yellow color:

const _ = db.command
db.collection('todos').where(_.or([
  {
    progress: _.lte(50)
  },
  {
    style: {
      color: _.in(['white', 'yellow'])
    }
  }
]))
.get({
  success: function(res) {
    console.log(res.data)
  }
})

For details, refer to the Database APIs documentation.