Exists query in Elasticsearch - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the exists query do in Elasticsearch?

The exists query finds documents that contain a specific field with any value. It checks if the field exists in the document.

beginner

How do you write an exists query to find documents with the field user?

Use this JSON structure:

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

intermediate

Can the exists query check for fields with empty or null values?

No, the exists query only checks if the field is present in the document, regardless of its value. If the field is missing or null, it will not match.

beginner

Why is the exists query useful in real-life search scenarios?

It helps find documents that have certain information filled in, like finding users who have provided an email address or products with a price set.

intermediate

What is the difference between exists query and checking for a field value in Elasticsearch?

The exists query checks only if the field is present, while checking for a field value looks for documents where the field has a specific value.

What does the Elasticsearch exists query check for?

A If a field is present in the document

B If a field has a specific value

C If a document contains a keyword

D If a document is deleted

The exists query checks if a field is present in the document, regardless of its value.

Which JSON snippet correctly uses the exists query to find documents with the field email?

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

B {"query": {"match": {"email": "exists"}}}

C {"query": {"term": {"email": "exists"}}}

D {"query": {"exists": {"value": "email"}}}

Option A correctly uses the exists query with the field key.

If a field is present but has a null value, will the exists query match it?

A Yes, it matches any present field

B Only if the field is indexed

C Only if the field is a string

D No, null values are treated as missing

Null values are treated as missing, so exists query will not match fields with null values.

Which use case fits best for the exists query?

A Find documents where a field is missing

B Find documents where a field has a specific value

C Find documents where a field is present

D Find documents with no fields

The exists query finds documents where a field is present.

What key do you use inside the exists query to specify the field name?

A value

B field

C name

D exists

The key field specifies which field to check for existence.

Explain how the exists query works in Elasticsearch and give a simple example.

Think about finding documents that have a certain piece of information filled in.

Describe a real-life scenario where using the exists query would be helpful.

Imagine you want to find all users who gave their phone number.

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