ClickHouse/docs/en/sql-reference/statements/select/index.md

title	toc_folder_title	toc_priority	toc_title
SELECT Query	SELECT	33	Overview

SELECT Query

SELECT queries perform data retrieval. By default, the requested data is returned to the client, while in conjunction with INSERT INTO it can be forwarded to a different table.

Syntax

[WITH expr_list|(subquery)]
SELECT [DISTINCT] expr_list
[FROM [db.]table | (subquery) | table_function] [FINAL]
[SAMPLE sample_coeff]
[ARRAY JOIN ...]
[GLOBAL] [ANY|ALL|ASOF] [INNER|LEFT|RIGHT|FULL|CROSS] [OUTER|SEMI|ANTI] JOIN (subquery)|table (ON <expr_list>)|(USING <column_list>)
[PREWHERE expr]
[WHERE expr]
[GROUP BY expr_list] [WITH TOTALS]
[HAVING expr]
[ORDER BY expr_list]
[LIMIT [offset_value, ]n BY columns]
[LIMIT [n, ]m]
[UNION ALL ...]
[INTO OUTFILE filename]
[FORMAT format]

All clauses are optional, except for the required list of expressions immediately after SELECT which is covered in more detail below.

Specifics of each optional clause are covered in separate sections, which are listed in the same order as they are executed:

• WITH clause
• FROM clause
• SAMPLE clause
• JOIN clause
• PREWHERE clause
• WHERE clause
• GROUP BY clause
• LIMIT BY clause
• HAVING clause
• SELECT clause
• DISTINCT clause
• LIMIT clause
• UNION ALL clause
• INTO OUTFILE clause
• FORMAT clause

SELECT Clause

Expressions specified in the SELECT clause are calculated after all the operations in the clauses described above are finished. These expressions work as if they apply to separate rows in the result. If expressions in the SELECT clause contain aggregate functions, then ClickHouse processes aggregate functions and expressions used as their arguments during the GROUP BY aggregation.

If you want to include all columns in the result, use the asterisk (*) symbol. For example, SELECT * FROM ....

To match some columns in the result with a re2 regular expression, you can use the COLUMNS expression.

COLUMNS('regexp')

For example, consider the table:

CREATE TABLE default.col_names (aa Int8, ab Int8, bc Int8) ENGINE = TinyLog

The following query selects data from all the columns containing the a symbol in their name.

SELECT COLUMNS('a') FROM col_names

┌─aa─┬─ab─┐
│  1 │  1 │
└────┴────┘

The selected columns are returned not in the alphabetical order.

You can use multiple COLUMNS expressions in a query and apply functions to them.

For example:

SELECT COLUMNS('a'), COLUMNS('c'), toTypeName(COLUMNS('c')) FROM col_names

┌─aa─┬─ab─┬─bc─┬─toTypeName(bc)─┐
│  1 │  1 │  1 │ Int8           │
└────┴────┴────┴────────────────┘

Each column returned by the COLUMNS expression is passed to the function as a separate argument. Also you can pass other arguments to the function if it supports them. Be careful when using functions. If a function doesn’t support the number of arguments you have passed to it, ClickHouse throws an exception.

For example:

SELECT COLUMNS('a') + COLUMNS('c') FROM col_names

Received exception from server (version 19.14.1):
Code: 42. DB::Exception: Received from localhost:9000. DB::Exception: Number of arguments for function plus doesn't match: passed 3, should be 2.

In this example, COLUMNS('a') returns two columns: aa and ab. COLUMNS('c') returns the bc column. The + operator can’t apply to 3 arguments, so ClickHouse throws an exception with the relevant message.

Columns that matched the COLUMNS expression can have different data types. If COLUMNS doesn’t match any columns and is the only expression in SELECT, ClickHouse throws an exception.

Asterisk

You can put an asterisk in any part of a query instead of an expression. When the query is analyzed, the asterisk is expanded to a list of all table columns (excluding the MATERIALIZED and ALIAS columns). There are only a few cases when using an asterisk is justified:

• When creating a table dump.
• For tables containing just a few columns, such as system tables.
• For getting information about what columns are in a table. In this case, set LIMIT 1. But it is better to use the DESC TABLE query.
• When there is strong filtration on a small number of columns using PREWHERE.
• In subqueries (since columns that aren’t needed for the external query are excluded from subqueries).

In all other cases, we don’t recommend using the asterisk, since it only gives you the drawbacks of a columnar DBMS instead of the advantages. In other words using the asterisk is not recommended.

Extreme Values

In addition to results, you can also get minimum and maximum values for the results columns. To do this, set the extremes setting to 1. Minimums and maximums are calculated for numeric types, dates, and dates with times. For other columns, the default values are output.

An extra two rows are calculated – the minimums and maximums, respectively. These extra two rows are output in JSON*, TabSeparated*, and Pretty* formats, separate from the other rows. They are not output for other formats.

In JSON* formats, the extreme values are output in a separate ‘extremes’ field. In TabSeparated* formats, the row comes after the main result, and after ‘totals’ if present. It is preceded by an empty row (after the other data). In Pretty* formats, the row is output as a separate table after the main result, and after totals if present.

Extreme values are calculated for rows before LIMIT, but after LIMIT BY. However, when using LIMIT offset, size, the rows before offset are included in extremes. In stream requests, the result may also include a small number of rows that passed through LIMIT.

Notes

You can use synonyms (AS aliases) in any part of a query.

The GROUP BY and ORDER BY clauses do not support positional arguments. This contradicts MySQL, but conforms to standard SQL. For example, GROUP BY 1, 2 will be interpreted as grouping by constants (i.e. aggregation of all rows into one).

Implementation Details

If the query omits the DISTINCT, GROUP BY and ORDER BY clauses and the IN and JOIN subqueries, the query will be completely stream processed, using O(1) amount of RAM. Otherwise, the query might consume a lot of RAM if the appropriate restrictions are not specified:

• max_memory_usage
• max_rows_to_group_by
• max_rows_to_sort
• max_rows_in_distinct
• max_bytes_in_distinct
• max_rows_in_set
• max_bytes_in_set
• max_rows_in_join
• max_bytes_in_join
• max_bytes_before_external_sort
• max_bytes_before_external_group_by

For more information, see the section “Settings”. It is possible to use external sorting (saving temporary tables to a disk) and external aggregation.

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