--- title: "substring" slug: "dev-database-reference-sql-functions-string-substring" updated: 2026-10-30T18:18:56Z published: 2026-10-30T18:18:56Z canonical: "docs.vastdata.com/dev-database-reference-sql-functions-string-substring" --- > ## 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. # substring *Applies to VAST Cluster 5.5.* *[Functions](/develop/docs/dev-database-reference-sql-functions-index) › [Text basics](/develop/docs/dev-database-reference-sql-functions-string-text-basics) › Parts of a string.* Returns part of a string: `length` characters from position `start`, or everything from `start` to the end. `substr` is the same function, `SUBSTRING(s FROM start FOR length)` is the standard SQL form of it, and `s[start:end]` and `s[i]` slice a string by position. ## Syntax ``` substring(s, start) substring(s, start, length) substr(s, start) substr(s, start, length) SUBSTRING(s FROM start) SUBSTRING(s FROM start FOR length) s[start:end] s[:end] s[start:] s[i] ``` | Argument | Type | Description | | --- | --- | --- | | `s` | `VARCHAR` | The string to take part of. | | `start` | `BIGINT` (any integer) | Where to start. Positions count from 1; a negative `start` counts from the end, `-1` being the last character. | | `length` | `BIGINT` (any integer) | How many characters to take. Leave it out to take everything to the end. | | `end` | integer | In a slice, the last position to take. It's included. | | `i` | integer | In `s[i]`, the one position to take; `-1` is the last character. | **Returns:** `VARCHAR`. **Aliases:** `substr`. `SUBSTRING(s FROM start FOR length)` is the same call as `substring(s, start, length)`. ## 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_substring`. 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 four products. Each SKU is a region (positions 1 and 2), a dash, a country or state code (positions 4 and 5), a dash and a five-digit serial number (positions 7 to 11); 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_substring"' 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, "") 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}.products" run(f"CREATE SCHEMA IF NOT EXISTS {SCHEMA}") run(f"CREATE TABLE IF NOT EXISTS {TABLE} (id BIGINT, sku VARCHAR, name VARCHAR)") run(f"INSERT INTO {TABLE} VALUES " "(1, 'EU-FR-00042', 'Café crème'), " "(2, 'US-NY-00107', 'Bagel'), " "(3, 'JP-13-00300', 'Matcha latte'), " "(4, NULL, NULL)") ``` ``` [] [] [(4,)] ``` ## What it does `substring` counts in Unicode code points. Think of the characters as numbered from 1, with an empty position 0 before the first one. With a positive `length`, `substring(s, start, length)` takes the positions from `start` to `start + length - 1`, and returns the characters that exist in that range: | Call | Positions taken | Result | | --- | --- | --- | | `substring('abcdef', 1, 2)` | 1 to 2 | `'ab'` | | `substring('abcdef', 2, 3)` with `FROM 2 FOR 3` | 2 to 4 | `'bcd'` | | `substr('abcdef', 3, 2)` | 3 to 4 | `'cd'` | | `substring('abcdef', 3)` with `FROM 3` | 3 to the end | `'cdef'` | | `substring('abcdef', 2, 0)` | none | `''` | | `substring('abcdef', 10)` | past the end | `''` | **A start of 0** is the empty position before the first character, and it counts towards `length`: `substring('abcdef', 0, 2)` takes positions 0 and 1, so it returns one character, `'a'`. **A negative start** counts from the end: `-1` is the last character, `-2` the one before it. `substring('abcdef', -2)` returns `'ef'`, and `substring('abcdef', -2, 1)` returns `'e'`. **A negative length** takes characters *before* `start`, not after it: `substring('abcdef', 2, -1)` takes the one character before position 2 and returns `'a'`. Use a positive length unless that's what you want. ### Slices: `s[start:end]` and `s[i]` The bracket forms take positions, not a length, and include both ends: | Slice | Positions taken | Result | Same as | | --- | --- | --- | --- | | `'abcdef'[2:3]` | 2 to 3 | `'bc'` | `substring('abcdef', 2, 2)` | | `'abcdef'[:2]` | the start to 2 | `'ab'` | `left('abcdef', 2)` | | `'abcdef'[4:]` | 4 to the end | `'def'` | `substring('abcdef', 4)` | | `'abcdef'[2]` | 2 | `'b'` | `substring('abcdef', 2, 1)` | | `'abcdef'[-1]` | the last | `'f'` | `right('abcdef', 1)` | | `'abcdef'[-2:]` | the second to last to the end | `'ef'` | `substring('abcdef', -2)` | | `'abcdef'[2:-2]` | 2 to the second to last | `'bcde'` | | | `'abcdef'[0]`, `'abcdef'[10]` | none | `''` | | A negative bound counts from the end, as a negative `start` does. A position that doesn't exist gives an empty string, not an error. Slices count code points too: on `'e\u0301x'`, `[1:1]` returns `'e'` without its accent and `[2]` returns the accent alone. Both forms return a `VARCHAR`. ### Code points, not what a reader sees A code point isn't always what a reader sees as one character. A letter followed by a combining accent (`e` and U+0301, displayed as é) is two code points, and so is a flag emoji. `substring('e\u0301x', 1, 1)` returns `'e'` without its accent, and a range can start on the accent or end in the middle of a flag: `substring('e\u0301 🇫🇷', 2, 3)` returns the accent alone, a space and the first half of the flag. To count what a reader sees, use [substring_grapheme](/develop/docs/dev-database-reference-sql-functions-string-substring-grapheme). ## NULLs and edge cases | Input | Result | | --- | --- | | `s` is NULL | `NULL` | | `start` is NULL | `NULL` | | `length` is NULL | `NULL` | | `''` (empty string) | `''` | | `substring('abcdef', 0, 2)` | `'a'` | | `substring('abcdef', -2)` | `'ef'` | | `substring('abcdef', -2, 1)` | `'e'` | | `substring('abcdef', 2, -1)` | `'a'` | | `substring('abcdef', 2, 0)` | `''` | | `substring('abcdef', 10)` | `''` | | `substring('e\u0301x', 1, 1)` (e, a combining accent, x) | `'e'` | | `substring('e\u0301 🇫🇷', 2, 3)` | the accent alone, a space and half the flag (3 code points) | | A `DECIMAL` position: `substring('abc', 1.5)` | Error: `No function matches the given name and argument types` | | `s` a number, date, boolean, `BLOB` or list | Error: `No function matches the given name and argument types` | 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-string-substring#set-up-the-examples): ``` run("SELECT substring(NULL, 2, 3), substring('abc', NULL), substring('abc', 1, NULL), substring('', 2, 3)") run("SELECT substring('abcdef', 0, 2), substring('abcdef', -2), substring('abcdef', -2, 1), " "substring('abcdef', 2, -1), substring('abcdef', 2, 0), substring('abcdef', 10), substring('e\u0301x', 1, 1)") run("SELECT substring('abcdef' FROM 2 FOR 3), substring('abcdef' FROM 3), substr('abcdef', 3, 2)") run("SELECT 'abcdef'[2:3], 'abcdef'[:2], 'abcdef'[4:], 'abcdef'[2], 'abcdef'[-1]") run("SELECT substring('e\u0301 \U0001F1EB\U0001F1F7', 2, 3), length(substring('e\u0301 \U0001F1EB\U0001F1F7', 2, 3))") run("SELECT substring('abc', 1.5)") run("SELECT substring(123, 2, 3)") ``` ``` [(None, None, None, '')] [('a', 'ef', 'e', 'a', '', '', 'e')] [('bcd', 'cdef', 'cd')] [('bc', 'ab', 'def', 'b', 'f')] [('́ 🇫', 3)] InternalError: Binder Error: No function matches the given name and argument types 'substring(STRING_LITERAL, DECIMAL(2,1))'. You might need to add explicit type casts. InternalError: Binder Error: No function matches the given name and argument types 'substring(INTEGER_LITERAL, INTEGER_LITERAL, INTEGER_LITERAL)'. You might need to add explicit type casts. ``` ## Examples The country or state code, three ways: with a start and a length, in the standard SQL form, and as a slice of positions 4 to 5: ``` run(f"SELECT id, substring(sku, 4, 2), SUBSTRING(sku FROM 4 FOR 2), sku[4:5] FROM {TABLE} ORDER BY id LIMIT 10") ``` ``` [(1, 'FR', 'FR', 'FR'), (2, 'NY', 'NY', 'NY'), (3, '13', '13', '13'), (4, None, None, None)] ``` The serial number, from position 7 to the end with `substr`, and as the last five characters with a negative start: ``` run(f"SELECT id, substr(sku, 7), substring(sku, -5) FROM {TABLE} ORDER BY id LIMIT 10") ``` ``` [(1, '00042', '00042'), (2, '00107', '00107'), (3, '00300', '00300'), (4, None, None)] ``` Keep only the products for France: ``` run(f"SELECT id, sku FROM {TABLE} WHERE substring(sku, 4, 2) = 'FR' ORDER BY id LIMIT 10") ``` ``` [(1, 'EU-FR-00042')] ``` ## 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 can evaluate a comparison of a column with a constant, but not a function of a column, so `substring()` and the slices are always computed by the Query Engine: - **In the `SELECT` list,** the scan reads the column and the Query Engine computes `substring()` for each row it returns. - **In `WHERE`,** the scan sends the column for every row, and a `FILTER` step in the Query Engine computes `substring()` and keeps the rows that match. No rows are skipped in storage. The plan of the `WHERE` example shows the `FILTER` step above the scan: ``` plan(f"SELECT id FROM {TABLE} WHERE substring(sku, 4, 2) = 'FR'") ``` ``` ┌───────────────────────────┐ │ PROJECTION │ │ ──────────────────── │ │ #1 │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ FILTER │ │ ──────────────────── │ │ ("substring"(sku, 4, 2) = │ │ 'FR') │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ VAST_SCAN │ │ ──────────────────── │ │ Function: VAST_SCAN │ │ │ │ Projections: │ │ sku │ │ id │ │ │ └───────────────────────────┘ ``` ## Make it faster - **Store the part you filter on.** If you often filter on one part of a value, such as the country in a SKU, split it into its own column when you write the rows: `country = 'FR'` is pushed down to VAST DataBase, so rows that don't match never leave storage. The plan for a raw column compared with a constant has no `FILTER` step: ``` plan(f"SELECT id FROM {TABLE} WHERE sku = 'EU-FR-00042'") ``` ``` ┌───────────────────────────┐ │ VAST_SCAN │ │ ──────────────────── │ │ Function: VAST_SCAN │ │ Projections: id │ │ │ └───────────────────────────┘ ``` - **A prefix test isn't faster.** `substring(sku, 1, 2) = 'EU'` and `sku LIKE 'EU%'` both run in a `FILTER` step: `LIKE 'EU%'` is rewritten to `prefix(sku, 'EU')`, which isn't pushed down either. - **Combine it with a pushed-down condition.** In `id > 100 AND substring(sku, 4, 2) = 'FR'`, `id > 100` is still evaluated in VAST DataBase, so `substring()` runs only on the rows that pass it. ## Limitations and errors - **`VARCHAR` only.** A number, a date, a boolean, a `BLOB` or a list fails with `No function matches the given name and argument types`. Cast a value to text first: `substring(CAST(n AS VARCHAR), 2, 3)`. For part of a list, use [list_slice](/develop/docs/dev-database-reference-sql-functions-lists-list-slice). - **Positions and lengths must be integers.** A `DECIMAL` such as `1.5` fails with `No function matches the given name and argument types 'substring(STRING_LITERAL, DECIMAL(2,1))'`; it isn't converted for you. Cast it to an integer first: `substring(s, CAST(x AS BIGINT))`. The cast rounds, it doesn't truncate: `CAST(1.5 AS BIGINT)` is 2, so `substring('abc', CAST(1.5 AS BIGINT))` returns `'bc'`. - **No `mid` or `substring_index`.** Both fail with `Scalar Function with name … does not exist!`. Use `substring(s, start, length)` for `mid`, and [split_part](/develop/docs/dev-database-reference-sql-functions-string-split-part) to take a part between separators. - **A negative length counts backwards.** `substring('abcdef', 2, -1)` returns `'a'`, not an error or an empty string. See [What it does](/develop/docs/dev-database-reference-sql-functions-string-substring#what-it-does). - **Code points, not characters.** `substring` can split a letter from its accent, or a flag in two. Use [substring_grapheme](/develop/docs/dev-database-reference-sql-functions-string-substring-grapheme) for text with accents or emoji. ``` run("SELECT substring(123, 2, 3)") run("SELECT substring(CAST(12345 AS VARCHAR), 2, 3), substring('abc', CAST(1.5 AS BIGINT))") ``` ``` InternalError: Binder Error: No function matches the given name and argument types 'substring(INTEGER_LITERAL, INTEGER_LITERAL, INTEGER_LITERAL)'. You might need to add explicit type casts. [('234', 'bc')] ``` ## Similar functions | Function | Takes | Result for `'abcdef'` | | --- | --- | --- | | `substring(s, 1, 2)`, `'abcdef'[1:2]` | code points by position | `'ab'` | | [left](/develop/docs/dev-database-reference-sql-functions-string-left) `(s, 2)` | the first `n` code points | `'ab'` | | [right](/develop/docs/dev-database-reference-sql-functions-string-right) `(s, 2)` | the last `n` code points | `'ef'` | | `substring(s, -2)` | code points from the `n`th from the end | `'ef'` | | [substring_grapheme](/develop/docs/dev-database-reference-sql-functions-string-substring-grapheme) | grapheme clusters by position: what a reader sees as characters | | | [split_part](/develop/docs/dev-database-reference-sql-functions-string-split-part) | a part between separators, by number | | | [regexp_extract](/develop/docs/dev-database-reference-sql-functions-string-regexp-extract) | the part that matches a pattern | | | [list_slice](/develop/docs/dev-database-reference-sql-functions-lists-list-slice) | elements of a list by position | | On `'e\u0301x'` (éx), `substring` and `substring_grapheme` differ: ``` run("SELECT substring('abcdef', 1, 2), 'abcdef'[1:2], left('abcdef', 2), right('abcdef', 2), substring('abcdef', -2)") run("SELECT substring('e\u0301x', 1, 1), substring_grapheme('e\u0301x', 1, 1)") ``` ``` [('ab', 'ab', 'ab', 'ef', 'ef')] [('e', 'é')] ``` Clean up: ``` run(f"DROP TABLE IF EXISTS {TABLE}") run(f"DROP SCHEMA IF EXISTS {SCHEMA}") conn.close() ``` ``` [] [] ``` ## See also - [Text basics](/develop/docs/dev-database-reference-sql-functions-string-text-basics): all the text functions. - [left](/develop/docs/dev-database-reference-sql-functions-string-left), [right](/develop/docs/dev-database-reference-sql-functions-string-right) and [substring_grapheme](/develop/docs/dev-database-reference-sql-functions-string-substring-grapheme). - [Where filters run](/develop/docs/dev-database-reference-sql-select#where-filters-run) and [pushdowns](/develop/docs/dev-database-reference-pushdowns). - [EXPLAIN](/develop/docs/dev-database-reference-sql-explain): how to read a plan.