Exists query in Elasticsearch - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of an Exists Query

What will be the output count of documents matched by this Elasticsearch query if the index contains documents with and without the field user?

Elasticsearch

{
  "query": {
    "exists": {
      "field": "user"
    }
  }
}

A The count of documents that have the field 'user' present

B The count of documents that do NOT have the field 'user'

C All documents in the index regardless of the 'user' field

D An error because 'exists' query requires a 'value' parameter

💡 Hint

The 'exists' query matches documents where the specified field is present.

The 'exists' query in Elasticsearch returns documents that contain the specified field. It does not match documents missing that field or all documents.

🧠 Conceptual

intermediate

Purpose of Exists Query

What is the main purpose of the Elasticsearch 'exists' query?

A To find documents where a specific field exists and is not null

B To find documents where a field equals a specific value

C To find documents where a field is missing

D To update documents that have a specific field

💡 Hint

Think about what 'exists' means in the context of fields in documents.

The 'exists' query is used to find documents that contain a particular field with any value, meaning the field is present and not null.

🔧 Debug

advanced

Why does this Exists Query fail?

Given this query, why does Elasticsearch return a syntax error?

Elasticsearch

{
  "query": {
    "exists": {
      "field" "user"
    }
  }
}

A The 'exists' query requires a 'value' parameter, which is missing

B Missing colon between 'field' and 'user' causes a JSON syntax error

C The field name 'user' must be in an array

D The 'exists' query cannot be used inside 'query' object

💡 Hint

Check the JSON syntax carefully around the 'field' key.

The JSON syntax requires a colon between keys and values. Missing the colon causes a syntax error.

❓ Predict Output

advanced

Output of Exists Query with Nested Fields

What documents will this query match if the index contains nested objects under 'comments' and some have 'author' fields?

Elasticsearch

{
  "query": {
    "nested": {
      "path": "comments",
      "query": {
        "exists": {
          "field": "comments.author"
        }
      }
    }
  }
}

A Documents that have the 'comments' field but no 'author' inside it

B Documents that have the 'author' field anywhere in the document

C Documents that have at least one 'comments' nested object with an 'author' field

D All documents regardless of 'comments' or 'author' fields

💡 Hint

The 'nested' query targets nested objects and the 'exists' query checks for field presence inside those objects.

The query matches documents where at least one nested 'comments' object contains the 'author' field.

🧠 Conceptual

expert

Behavior of Exists Query on Null and Empty Fields

Which statement correctly describes how the Elasticsearch 'exists' query treats fields with null or empty string values?

A Empty string values cause the 'exists' query to fail with an error

B Fields with null values match the 'exists' query because the field is present

C The 'exists' query matches only fields with non-empty string values

D Fields with null values are considered missing and will NOT match the 'exists' query

💡 Hint

Think about how Elasticsearch indexes null values and what 'exists' means.

Elasticsearch does not index fields with null values, so they are treated as missing and do not match the 'exists' query. Empty strings are indexed and do match.

Practice

1. What does the exists query do in Elasticsearch?

easy

A. Finds documents where a specific field is present

B. Finds documents where a field has a specific value

C. Deletes documents missing a field

D. Updates documents with a missing field

Solution

1. Step 1: Understand the purpose of the exists query

   The exists query checks if a field exists in documents, filtering those that have the field.

2. Step 2: Compare options with the query's function

   Only Finds documents where a specific field is present correctly describes this behavior; others describe different operations.

3. Final Answer:

   Finds documents where a specific field is present -> Option A

4. Quick Check:

   Exists query = finds documents with field [OK]

Hint: Exists query checks if a field is present in documents [OK]

Common Mistakes:

• Confusing exists query with term query
• Thinking it deletes or updates documents
• Assuming it checks field values

2. Which of the following is the correct syntax for an exists query in Elasticsearch?

easy

A. {"query": {"exists": {"field_name": "user"}}}

B. {"query": {"exists": "user"}}

C. {"query": {"exists": {"value": "user"}}}

D. {"query": {"exists": {"field": "user"}}}

Solution

1. Step 1: Recall the correct JSON structure for exists query

   The exists query requires a "field" key inside its object specifying the field name.

2. Step 2: Match options to correct syntax

   {"query": {"exists": {"field": "user"}}} matches the correct syntax: {"exists": {"field": "user"}}. Others use wrong keys or structure.

3. Final Answer:

   {"query": {"exists": {"field": "user"}}} -> Option D

4. Quick Check:

   Exists query syntax uses "field" key [OK]

Hint: Exists query needs "field" key inside its object [OK]

Common Mistakes:

• Using "value" or "field_name" instead of "field"
• Omitting the object braces around field
• Passing field as a string directly

3. Given the following Elasticsearch query, what documents will it return?

{"query": {"exists": {"field": "email"}}}

medium

A. Documents where the email field exists

B. Documents where the email field is missing

C. Documents where the email field is empty

D. All documents regardless of email field

Solution

1. Step 1: Analyze the query structure

   The query uses an exists query on the field "email", which filters documents having that field.

2. Step 2: Understand what exists means for documents

   Documents returned must have the "email" field present, regardless of its value.

3. Final Answer:

   Documents where the email field exists -> Option A

4. Quick Check:

   Exists query returns docs with field present [OK]

Hint: Exists query returns docs with the field present only [OK]

Common Mistakes:

• Thinking it returns docs missing the field
• Assuming empty fields are excluded
• Believing it returns all documents

4. Identify the error in this Elasticsearch exists query:

{"query": {"exists": {"field_name": "status"}}}

medium

A. The field value should be an array, not a string

B. The query should use "match" instead of "exists"

C. The key should be "field", not "field_name"

D. The query is missing a "filter" wrapper

Solution

1. Step 1: Check the key used inside exists query

   The exists query requires the key "field" to specify the field name.

2. Step 2: Compare the given key with the correct one

   The query uses "field_name" which is incorrect and causes an error.

3. Final Answer:

   The key should be "field", not "field_name" -> Option C

4. Quick Check:

   Exists query key = "field" [OK]

Hint: Use "field" key inside exists query, not "field_name" [OK]

Common Mistakes:

• Using incorrect key names inside exists query
• Confusing exists with match query
• Adding unnecessary wrappers

5. You want to find all documents where the field phone exists but is not empty or null. Which Elasticsearch query best achieves this?

hard

A. {"query": {"bool": {"must": [{"exists": {"field": "phone"}}, {"term": {"phone": ""}}]}}}

B. {"query": {"bool": {"must": [{"exists": {"field": "phone"}}, {"script": {"script": "doc['phone'].value != ''"}}]}}}

C. {"query": {"exists": {"field": "phone"}}}

D. {"query": {"exists": {"field": "phone"}, "term": {"phone": ""}}}

Solution

1. Step 1: Understand the requirement

   We want documents where "phone" exists and is not empty or null.

2. Step 2: Evaluate each option

   {"query": {"exists": {"field": "phone"}}} only checks existence, not emptiness. {"query": {"bool": {"must": [{"exists": {"field": "phone"}}, {"term": {"phone": ""}}]}}} incorrectly requires phone to be empty. {"query": {"exists": {"field": "phone"}, "term": {"phone": ""}}} has invalid syntax combining exists and term at same level. {"query": {"bool": {"must": [{"exists": {"field": "phone"}}, {"script": {"script": "doc['phone'].value != ''"}}]}}} uses a bool must with exists and a script checking phone is not empty, correctly filtering as needed.

3. Final Answer:

   {"query": {"bool": {"must": [{"exists": {"field": "phone"}}, {"script": {"script": "doc['phone'].value != ''"}}]}}} -> Option B

4. Quick Check:

   Exists + script to exclude empty = {"query": {"bool": {"must": [{"exists": {"field": "phone"}}, {"script": {"script": "doc['phone'].value != ''"}}]}}} [OK]

Hint: Combine exists with script to exclude empty values [OK]

Common Mistakes:

• Using exists alone without checking empty values
• Incorrectly combining queries at same level
• Assuming term query with empty string works for exclusion