---
title: "union_value"
slug: "dev-database-reference-sql-functions-utility-union-value"
updated: 2026-10-30T18:18:46Z
published: 2026-10-30T18:18:46Z
canonical: "docs.vastdata.com/dev-database-reference-sql-functions-utility-union-value"
---

> ## 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.

# union_value

*Applies to VAST Cluster 5.5.*

*[Functions](/develop/docs/dev-database-reference-sql-functions-index) › [List, struct and map functions](/develop/docs/dev-database-reference-sql-functions-lists-and-structs) › Union values.* Builds a value of the `UNION` type from one value and a name for it, its tag: `union_value(k := 1)` is a `UNION(k INTEGER)` holding `1`. A union value holds one of several named members, and its tag says which. Cast it to a wider `UNION` type to give it more members. Union values exist only inside a query: a table can't have a `UNION` column.

## Syntax

```
union_value(tag := value)
```

| Argument | Type | Description |
| --- | --- | --- |
| `tag` | A name | The member's name. It's required, written as a named argument with `:=`. |
| `value` | Any type | The member's value. Its type becomes the member's type. |

**Returns:** `UNION(tag type)`, a union with one member: `union_value(k := 1)` is a `UNION(k INTEGER)`.

## Set up the examples

The examples connect with the ADBC driver, as in [VAST Query Engine (ADBC)](/develop/docs/dev-database-integrations-adbc), and work in a schema called `docs_sql_fn_utility_union_value`. Set `QE_ENDPOINT`, `AWS_ACCESS_KEY_ID`, `AWS_SECRET_ACCESS_KEY` and `VASTDB_BUCKET` ([SQL reference](/develop/docs/dev-database-reference-sql-index#example-connect-and-use-a-transaction)), and run the blocks in order in one Python session. Below each block is the output you should see.

This block connects, defines the `run()` and `plan()` helpers, and creates a table of five sensor readings. A reading is either a number or a text status; the last row has no values:

```
import os
import re

import adbc_driver_manager
import adbc_driver_manager.dbapi as dbapi
import adbc_driver_vastdb

BUCKET = os.environ["VASTDB_BUCKET"]
SCHEMA = f'"{BUCKET}/docs_sql_fn_utility_union_value"'

conn = dbapi.connect(
    driver=adbc_driver_vastdb.get_driver_path(),
    db_kwargs={
        "vast.db.endpoint": os.environ["QE_ENDPOINT"],
        "vast.db.access_key": os.environ["AWS_ACCESS_KEY_ID"],
        "vast.db.secret_key": os.environ["AWS_SECRET_ACCESS_KEY"],
    },
    autocommit=True,
)

def server_message(e):
    """The server's message in a driver error, without the request ID or the echoed statement."""
    text = re.sub(r"\[req=0x[0-9a-f]+\]: ", "", str(e))
    found = (re.search(r'message: "(.*?)", request_id', text, re.S)
             or re.search(r"description: (.*)\)", text, re.S))
    message = found.group(1) if found else text.strip().splitlines()[-1]
    message = re.sub(r"^Plan query: '.*?' failed with: ", "", message, flags=re.S)
    message = re.split(r"\\n|\n", message)[0].replace('\\"', '"')
    return message.replace(BUCKET, "<bucket>")

def run(sql):
    """Run one statement. Print the rows it returns, or the error."""
    try:
        with conn.cursor() as cur:
            cur.execute(sql)
            rows = cur.fetchall() if cur.description else []
    except (adbc_driver_manager.Error, OSError) as e:
        print(f"{type(e).__name__}: {server_message(e)}")
    else:
        print(rows)

def plan(sql):
    """Print the plan without its ~N Rows lines: the table's row count, which can still be 0 just after an insert."""
    with conn.cursor() as cur:
        cur.execute("EXPLAIN " + sql)
        steps = cur.fetchall()[0][1]
    print("\n".join(line for line in steps.splitlines() if "Rows" not in line))

TABLE = f"{SCHEMA}.readings"
run(f"CREATE SCHEMA IF NOT EXISTS {SCHEMA}")
run(f"CREATE TABLE IF NOT EXISTS {TABLE} (id BIGINT, num BIGINT, status VARCHAR)")
run(f"INSERT INTO {TABLE} VALUES "
    "(1, 21, NULL), "
    "(2, NULL, 'offline'), "
    "(3, -4, NULL), "
    "(4, NULL, 'calibrating'), "
    "(5, NULL, NULL)")
```

```
[]
[]
[(5,)]
```

## What it does

`union_value(tag := value)` wraps `value` in a union whose only member is `tag`:

| Call | Type | The driver returns |
| --- | --- | --- |
| `union_value(k := 1)` | `UNION(k INTEGER)` | `1` |
| `union_value(k := &apos;a&apos;)` | `UNION(k VARCHAR)` | `&apos;a&apos;` |
| `union_value(k := [1, 2])` | `UNION(k INTEGER[])` | `[1, 2]` |

To hold one of several members, cast the result to a `UNION` type that lists them all: `union_value(k := 1)::UNION(k INTEGER, s VARCHAR)`. Read a member back with [union_extract](/develop/docs/dev-database-reference-sql-functions-utility-union-extract), which returns `NULL` for a member the value doesn't hold. `union_tag(u)::VARCHAR` returns the tag a value holds.

## NULLs and edge cases

| Input | Result |
| --- | --- |
| `union_value(k := NULL)` | A `UNION(k INTEGER)` holding `NULL`: the driver returns `None`, but `IS NOT NULL` is true for it |
| `union_value(42)`, without a tag | Error: `Need named argument for union tag, e.g. UNION_VALUE(a := b)` |
| `union_value(k := 1, m := 2)` | Error: `union_value takes exactly one argument` |
| A table column of a `UNION` type | Error: `UNION type is not supported in table definitions. Column &quot;x&quot; has type: UNION(k INTEGER, s VARCHAR)` |

The same cases, as the cluster returns them. They use the table and helpers from [Set up the examples](/develop/docs/dev-database-reference-sql-functions-utility-union-value#set-up-the-examples):

```
run("SELECT union_value(k := NULL), union_value(k := 1), union_value(k := 'a'), union_value(k := [1, 2])")
run("SELECT typeof(union_value(k := 1)), typeof(union_value(k := 'a'))")
run(f"SELECT count(*) FROM {TABLE} WHERE union_value(num := num) IS NOT NULL")
run("SELECT union_value(42)")
run("SELECT union_value(k := 1, m := 2)")
run(f"CREATE TABLE {SCHEMA}.unions (id BIGINT, x UNION(k INTEGER, s VARCHAR))")
```

```
[(None, 1, 'a', [1, 2])]
[('UNION(k INTEGER)', 'UNION(k VARCHAR)')]
[(5,)]
InternalError: Binder Error: Need named argument for union tag, e.g. UNION_VALUE(a := b)
InternalError: Binder Error: union_value takes exactly one argument
InternalError: Binder Error: UNION type is not supported in table definitions. Column "x" has type: UNION(k INTEGER, s VARCHAR)
```

## Examples

Wrap each numeric reading in a union with the tag `num`:

```
run(f"SELECT id, union_value(num := num), typeof(union_value(num := num)) FROM {TABLE} ORDER BY id LIMIT 10")
```

```
[(1, 21, 'UNION(num BIGINT)'), (2, None, 'UNION(num BIGINT)'), (3, -4, 'UNION(num BIGINT)'), (4, None, 'UNION(num BIGINT)'), (5, None, 'UNION(num BIGINT)')]
```

One value per reading, a number or a status, of one `UNION(num BIGINT, status VARCHAR)` type. The query returns the tag each row holds and both members, one of which is `NULL`:

```
run(f"SELECT id, union_tag(r)::VARCHAR, union_extract(r, 'num'), union_extract(r, 'status') FROM ("
    "SELECT id, CASE WHEN num IS NOT NULL "
    "THEN union_value(num := num)::UNION(num BIGINT, status VARCHAR) "
    "ELSE union_value(status := status)::UNION(num BIGINT, status VARCHAR) END AS r "
    f"FROM {TABLE}) ORDER BY id LIMIT 10")
```

```
[(1, 'num', 21, None), (2, 'status', None, 'offline'), (3, 'num', -4, None), (4, 'status', None, 'calibrating'), (5, 'status', None, None)]
```

## How it runs in VAST

The Query Engine runs on the cluster's CNodes, and a query on a table starts with a scan in VAST DataBase (`VAST_SCAN`). VAST DataBase stores no `UNION` columns and can't evaluate a function of a column, so `union_value()` is always computed by the Query Engine:

- **In the `SELECT` list,** the scan reads the column and the Query Engine builds the union for each row it returns.
- **In `WHERE`,** the scan sends the column for every row, and a `FILTER` step in the Query Engine builds the union and keeps the rows that match. No rows are skipped in storage.

The plan of a `WHERE` condition on a union shows the `FILTER` step above the scan. This condition keeps every row, also the rows whose `num` is `NULL`, because a union that holds `NULL` is not itself `NULL`:

```
plan(f"SELECT id FROM {TABLE} WHERE union_value(num := num) IS NOT NULL")
```

```
┌───────────────────────────┐
│         PROJECTION        │
│    ────────────────────   │
│             #1            │
│                           │
└─────────────┬─────────────┘
┌─────────────┴─────────────┐
│           FILTER          │
│    ────────────────────   │
│  (union_value(num) IS NOT │
│            NULL)          │
│                           │
└─────────────┬─────────────┘
┌─────────────┴─────────────┐
│         VAST_SCAN         │
│    ────────────────────   │
│    Function: VAST_SCAN    │
│                           │
│        Projections:       │
│            num            │
│             id            │
│                           │
└───────────────────────────┘
```

## Make it faster

- **Filter on the raw columns.** To keep the rows that have a number, test the column, `num IS NOT NULL`, not the union. A condition on a raw column is pushed down to VAST DataBase, so rows that don't match never leave storage. Compare its plan with the one above:

```
plan(f"SELECT id FROM {TABLE} WHERE num IS NOT NULL")
```

```
┌───────────────────────────┐
│         VAST_SCAN         │
│    ────────────────────   │
│    Function: VAST_SCAN    │
│      Projections: id      │
│                           │
└───────────────────────────┘
```

- **Build unions last,** in the `SELECT` list or a subquery, from rows already filtered on their columns.

## Limitations and errors

- **No `UNION` columns.** `CREATE TABLE` with a `UNION` column fails with `UNION type is not supported in table definitions`. To store a value that can be one of several types, keep one column per member, as the example table does, or a [struct](/develop/docs/dev-database-reference-sql-functions-structs-struct-pack) with one field per member.
- **One member per call, with a tag.** See [NULLs and edge cases](/develop/docs/dev-database-reference-sql-functions-utility-union-value#nulls-and-edge-cases).
- **Read the tag as text.** The driver can't read the raw result of `union_tag()`; cast it: `union_tag(u)::VARCHAR`.

```
run("SELECT union_tag(union_value(k := 1))::VARCHAR")
```

```
[('k',)]
```

## Similar functions

| Function | Does | Example | Result |
| --- | --- | --- | --- |
| `union_value` | Builds a union with one member | `union_value(k := 1)` | `1`, a `UNION(k INTEGER)` |
| [union_extract](/develop/docs/dev-database-reference-sql-functions-utility-union-extract) | Reads one member of a union | `union_extract(union_value(k := 1), &apos;k&apos;)` | `1` |
| `union_tag` | Returns the tag a union holds | `union_tag(union_value(k := 1))::VARCHAR` | `&apos;k&apos;` |
| [struct_pack](/develop/docs/dev-database-reference-sql-functions-structs-struct-pack) | Builds a struct, which holds all its fields at once |  |  |

```
run("SELECT union_value(k := 1), union_extract(union_value(k := 1), 'k'), union_tag(union_value(k := 1))::VARCHAR")
run("SELECT struct_pack(k := 1, s := 'a')")
```

```
[(1, 1, 'k')]
[({'k': 1, 's': 'a'},)]
```

Clean up:

```
run(f"DROP TABLE IF EXISTS {SCHEMA}.unions")
run(f"DROP TABLE IF EXISTS {TABLE}")
run(f"DROP SCHEMA IF EXISTS {SCHEMA}")
conn.close()
```

```
[]
[]
[]
```

## See also

- [union_extract](/develop/docs/dev-database-reference-sql-functions-utility-union-extract).
- [List, struct and map functions](/develop/docs/dev-database-reference-sql-functions-lists-and-structs).
- [Data types](/develop/docs/dev-database-reference-sql-data-types).
- [Where filters run](/develop/docs/dev-database-reference-sql-select#where-filters-run) and [pushdowns](/develop/docs/dev-database-reference-pushdowns).