---
title: "Parsing and JSON functions"
slug: "dev-database-reference-sql-functions-json"
updated: 2026-10-30T18:19:06Z
published: 2026-10-30T18:19:06Z
canonical: "docs.vastdata.com/dev-database-reference-sql-functions-json"
---

> ## Documentation Index
> Fetch the complete documentation index at: https://docs.vastdata.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Parsing and JSON functions

*Applies to VAST Cluster 5.5.*

Reading, checking, converting and building JSON text.

Each function has its own page: syntax, what it does, NULLs and edge cases, examples with their output, how it runs in VAST, how to make it faster, limitations and errors, and similar functions. Names that the VAST Query Engine doesn't have are in [all functions A-Z](/develop/docs/dev-database-reference-sql-functions-a-z).

## Check and inspect

| Function | What it does |
| --- | --- |
| [json_array_length](/develop/docs/dev-database-reference-sql-functions-json-json-array-length) | Count the elements of a JSON array, or of an array at a path; anything that isn't an array counts 0. |
| [json_exists](/develop/docs/dev-database-reference-sql-functions-json-json-exists) | Check whether a path exists in a JSON document, including keys whose value is JSON null. |
| [json_keys](/develop/docs/dev-database-reference-sql-functions-json-json-keys) | List the keys of a JSON object, or of an object at a path, in document order. |
| [json_type](/develop/docs/dev-database-reference-sql-functions-json-json-type) | Return the type of a JSON document, or of the value at a path: OBJECT, ARRAY, VARCHAR, a number type, BOOLEAN or NULL. |
| [json_valid](/develop/docs/dev-database-reference-sql-functions-json-json-valid) | Check whether text is valid JSON, without failing on invalid input. |
| [json_contains](/develop/docs/dev-database-reference-sql-functions-json-json-contains) | Test whether a JSON document contains a value, an object's key/value pairs or an array's elements anywhere inside it. |
| [json_structure](/develop/docs/dev-database-reference-sql-functions-json-json-structure) | Describe the shape of a JSON document, with a SQL type name for each value; the result can be used as the structure argument of json_transform. |

## Read values

| Function | What it does |
| --- | --- |
| [json_extract_string](/develop/docs/dev-database-reference-sql-functions-json-json-extract-string) (also `json_extract_path_text`, `->>`) | Read the value at a path in a JSON document as plain text; json_extract_path_text and the ->> operator are the same function. |
| [json_extract](/develop/docs/dev-database-reference-sql-functions-json-json-extract) (also `json_extract_path`, `->`) | Read the value at a path in a JSON document, as JSON text; json_extract_path and the -> operator are the same function. |
| [json_value](/develop/docs/dev-database-reference-sql-functions-json-json-value) | Read the scalar value at a path in a JSON document, as JSON text; objects and arrays return NULL. |

## Build JSON

| Function | What it does |
| --- | --- |
| [array_to_json](/develop/docs/dev-database-reference-sql-functions-json-array-to-json) | Convert a list to a JSON array; any other argument type fails. |
| [json_quote](/develop/docs/dev-database-reference-sql-functions-json-json-quote) | Convert a SQL value to JSON text, quoting and escaping strings; it gives the same results as to_json. |
| [row_to_json](/develop/docs/dev-database-reference-sql-functions-json-row-to-json) | Convert a struct to a JSON object, with the field names as keys; any other argument type fails. |
| [to_json](/develop/docs/dev-database-reference-sql-functions-json-to-json) | Convert any SQL value (a number, string, date, list, struct or map) to JSON text. |
| [json_array](/develop/docs/dev-database-reference-sql-functions-json-json-array) | Build a JSON array from a list of values of any type, with how each type is written and how NULLs become null. |
| [json_merge_patch](/develop/docs/dev-database-reference-sql-functions-json-json-merge-patch) | Apply JSON merge patches to a document: add, replace and remove keys, merge nested objects, and what NULLs and non-object documents do. |
| [json_object](/develop/docs/dev-database-reference-sql-functions-json-json-object) | Build a JSON object from key-value pairs, with how keys and values are written, NULL keys and values, and the errors. |
| [json_pretty](/develop/docs/dev-database-reference-sql-functions-json-json-pretty) | Write a JSON document over several lines, indented by four spaces, with the result type, NULLs and the errors for invalid JSON. |
| [json](/develop/docs/dev-database-reference-sql-functions-json-json) | Parse JSON text and write it back in compact form: what changes, how numbers and escapes are written, and the errors for invalid JSON. |

## Convert to SQL types

| Function | What it does |
| --- | --- |
| [json_transform_strict](/develop/docs/dev-database-reference-sql-functions-json-json-transform-strict) (also `from_json_strict`) | Convert a JSON document to a struct, list or value of SQL types, failing on a missing key or a value that doesn't convert; from_json_strict is the same function. |
| [json_transform](/develop/docs/dev-database-reference-sql-functions-json-json-transform) (also `from_json`) | Convert a JSON document to a struct, list or value of SQL types in one pass, with NULL for fields that are missing or don't convert; from_json is the same function. |

## SQL as JSON

| Function | What it does |
| --- | --- |
| [json_deserialize_sql](/develop/docs/dev-database-reference-sql-functions-json-json-deserialize-sql) | Turn a JSON syntax tree from json_serialize_sql back into SQL text, how the text is normalised, and the errors. |
| [json_serialize_sql](/develop/docs/dev-database-reference-sql-functions-json-json-serialize-sql) | Parse the text of a SELECT statement into its syntax tree as JSON, with the options, the error object and how to read fields from the result. |

## See also

- [SQL functions](/develop/docs/dev-database-reference-sql-functions-index) and [all functions A-Z](/develop/docs/dev-database-reference-sql-functions-a-z).