TinyDB - The exists() Query

TinyDB provides an advanced query called exists() that checks the existence of data in a JSON file. The exists() query actually tests the availabipty of a subfield data from a JSON file. The exists() query works on the basis of a Boolean condition. If the subfield exists (i.e., BOOLEAN TRUE), it will fetch the data accordingly from the JSON file, otherwise it will return a blank value.

Syntax

The syntax of TinyDB exists() is as follows −

db.search(Query().field.exists())

Here, field represents the part of data that we want to access. Query() is the object created from the JSON table student.

We will use the same student database that we have used in all the previous chapters.

Example 1

Let s use the TinyDB exists() query for the field named subject −

db.search(student.subject.exists())

This query will fetch all the rows because all the rows have the "subject" field −

[
   {
      "roll_number":1,
      "st_name":"elen",
      "mark":250,
      "subject":"TinyDB",
      "address":"delhi"
   },
   {
      "roll_number":2,
      "st_name":"Ram",
      "mark":[
         250,
         280
      ],
      "subject":[
         "TinyDB",
         "MySQL"
      ],
      "address":"delhi"
   },
   {
      "roll_number":3,
      "st_name":"kevin",
      "mark":[
         180,
         200
      ],
      "subject":[
         "oracle",
         "sql"
      ],
      "address":"keral"
   },
   {
      "roll_number":4,
      "st_name":"lakan",
      "mark":200,
      "subject":"MySQL",
      "address":"mumbai"
   },
   {
      "roll_number":5,
      "st_name":"karan",
      "mark":275,
      "subject":" TinyDB ",
      "address":"benglore"
   }
]

Example 2

Now let s use the exists() query for the address field −

db.search(student.address.exists())

It will fetch the following rows −

[
   {
      "roll_number":1,
      "st_name":"elen",
      "mark":250,
      "subject":"TinyDB",
      "address":"delhi"
   },
   {
      "roll_number":2,
      "st_name":"Ram",
      "mark":[
         250,
         280
      ],
      "subject":[
         "TinyDB",
         "MySQL"
      ],
      "address":"delhi"
   },
   {
      "roll_number":3,
      "st_name":"kevin",
      "mark":[
         180,
         200
      ],
      "subject":[
         "oracle",
         "sql"
      ],
      "address":"keral"
   },
   {
      "roll_number":4,
      "st_name":"lakan",
      "mark":200,
      "subject":"MySQL",
      "address":"mumbai"
   },
   {
      "roll_number":5,
      "st_name":"karan",
      "mark":275,
      "subject":" TinyDB ",
      "address":"benglore"
   }
]

Example 3

Now, let s try the exists() query for a field that is not available −

db.search(student.city.exists())

Since none of the rows in the given table has a field called "city", the above exists() query will return a blank value −

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