ClickHouse/docs/en/query_language/syntax.md

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# Syntax
There are two types of parsers in the system: the full SQL parser (a recursive descent parser), and the data format parser (a fast stream parser).
In all cases except the INSERT query, only the full SQL parser is used.
The INSERT query uses both parsers:
``` sql
INSERT INTO t VALUES (1, 'Hello, world'), (2, 'abc'), (3, 'def')
```
The `INSERT INTO t VALUES` fragment is parsed by the full parser, and the data `(1, 'Hello, world'), (2, 'abc'), (3, 'def')` is parsed by the fast stream parser.
Data can have any format. When a query is received, the server calculates no more than `max_query_size` bytes of the request in RAM (by default, 1 MB), and the rest is stream parsed.
This means the system doesn't have problems with large INSERT queries, like MySQL does.
When using the Values format in an INSERT query, it may seem that data is parsed the same as expressions in a SELECT query, but this is not true. The Values format is much more limited.
Next we will cover the full parser. For more information about format parsers, see the section "Formats".
## Spaces
There may be any number of space symbols between syntactical constructions (including the beginning and end of a query). Space symbols include the space, tab, line feed, CR, and form feed.
## Comments
SQL-style and C-style comments are supported.
SQL-style comments: from `--` to the end of the line. The space after `--` can be omitted.
Comments in C-style: from `/*` to `*/`. These comments can be multiline. Spaces are not required here, either.
## Keywords {#syntax-keywords}
Keywords (such as `SELECT`) are not case-sensitive. Everything else (column names, functions, and so on), in contrast to standard SQL, is case-sensitive.
Keywords are not reserved (they are just parsed as keywords in the corresponding context). If you use [identifiers](#syntax-identifiers) the same as the keywords, enclose them into quotes. For example, the query `SELECT "FROM" FROM table_name` is valid if the table `table_name` has column with the name `"FROM"`.
## Identifiers {#syntax-identifiers}
Identifiers are:
- Cluster, database, table, partition and column names;
- Functions;
- Data types;
- [Expression aliases](#syntax-expression_aliases).
Identifiers can be quoted or non-quoted. It is recommended to use non-quoted identifiers.
Non-quoted identifiers must match the regex `^[a-zA-Z_][0-9a-zA-Z_]*$` and can not be equal to [keywords](#syntax-keywords). Examples: `x, _1, X_y__Z123_.`
If you want to use identifiers the same as keywords or you want to use other symbols in identifiers, quote it using double quotes or backticks, for example, `"id"`, `` `id` ``.
## Literals
There are: numeric, string, compound and `NULL` literals.
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### Numeric
A numeric literal tries to be parsed:
- First as a 64-bit signed number, using the [strtoull](https://en.cppreference.com/w/cpp/string/byte/strtoul) function.
- If unsuccessful, as a 64-bit unsigned number, using the [strtoll](https://en.cppreference.com/w/cpp/string/byte/strtol) function.
- If unsuccessful, as a floating-point number using the [strtod](https://en.cppreference.com/w/cpp/string/byte/strtof) function.
- Otherwise, an error is returned.
The corresponding value will have the smallest type that the value fits in.
For example, 1 is parsed as `UInt8`, but 256 is parsed as `UInt16`. For more information, see [Data types](../data_types/index.md).
Examples: `1`, `18446744073709551615`, `0xDEADBEEF`, `01`, `0.1`, `1e100`, `-1e-100`, `inf`, `nan`.
### String
Only string literals in single quotes are supported. The enclosed characters can be backslash-escaped. The following escape sequences have a corresponding special value: `\b`, `\f`, `\r`, `\n`, `\t`, `\0`, `\a`, `\v`, `\xHH`. In all other cases, escape sequences in the format `\c`, where `c` is any character, are converted to `c`. This means that you can use the sequences `\'`and`\\`. The value will have the String type.
The minimum set of characters that you need to escape in string literals: `'` and `\`. Single quote can be escaped with the single quote, literals `'It\'s'` and `'It''s'` are equal.
### Compound
Constructions are supported for arrays: `[1, 2, 3]` and tuples: `(1, 'Hello, world!', 2)`..
Actually, these are not literals, but expressions with the array creation operator and the tuple creation operator, respectively.
An array must consist of at least one item, and a tuple must have at least two items.
Tuples have a special purpose for use in the `IN` clause of a `SELECT` query. Tuples can be obtained as the result of a query, but they can't be saved to a database (with the exception of [Memory](../operations/table_engines/memory.md) tables).
### NULL {#null-literal}
Indicates that the value is missing.
WIP on docs (#3813) * CLICKHOUSE-4063: less manual html @ index.md * CLICKHOUSE-4063: recommend markdown="1" in README.md * CLICKHOUSE-4003: manually purge custom.css for now * CLICKHOUSE-4064: expand <details> before any print (including to pdf) * CLICKHOUSE-3927: rearrange interfaces/formats.md a bit * CLICKHOUSE-3306: add few http headers * Remove copy-paste introduced in #3392 * Hopefully better chinese fonts #3392 * get rid of tabs @ custom.css * Apply comments and patch from #3384 * Add jdbc.md to ToC and some translation, though it still looks badly incomplete * minor punctuation * Add some backlinks to official website from mirrors that just blindly take markdown sources * Do not make fonts extra light * find . -name '*.md' -type f | xargs -I{} perl -pi -e 's//g' {} * find . -name '*.md' -type f | xargs -I{} perl -pi -e 's/ sql/g' {} * Remove outdated stuff from roadmap.md * Not so light font on front page too * Refactor Chinese formats.md to match recent changes in other languages * Update some links on front page * Remove some outdated comment * Add twitter link to front page * More front page links tuning * Add Amsterdam meetup link * Smaller font to avoid second line * Add Amsterdam link to README.md * Proper docs nav translation * Back to 300 font-weight except Chinese * fix docs build * Update Amsterdam link * remove symlinks * more zh punctuation * apply lost comment by @zhang2014 * Apply comments by @zhang2014 from #3417 * Remove Beijing link * rm incorrect symlink * restore content of docs/zh/operations/table_engines/index.md * CLICKHOUSE-3751: stem terms while searching docs * CLICKHOUSE-3751: use English stemmer in non-English docs too * CLICKHOUSE-4135 fix * Remove past meetup link * Add blog link to top nav * Add ContentSquare article link * Add form link to front page + refactor some texts * couple markup fixes * minor * Introduce basic ODBC driver page in docs * More verbose 3rd party libs disclaimer * Put third-party stuff into a separate folder * Separate third-party stuff in ToC too * Update links * Move stuff that is not really (only) a client library into a separate page * Add clickhouse-hdfs-loader link * Some introduction for "interfaces" section * Rewrite tcp.md * http_interface.md -> http.md * fix link * Remove unconvenient error for now * try to guess anchor instead of failing * remove symlink * Remove outdated info from introduction * remove ru roadmap.md * replace ru roadmap.md with symlink * Update roadmap.md * lost file * Title case in toc_en.yml * Sync "Functions" ToC section with en * Remove reference to pretty old ClickHouse release from docs * couple lost symlinks in fa * Close quote in proper place * Rewrite en/getting_started/index.md * Sync en<>ru getting_started/index.md * minor changes * Some gui.md refactoring * Translate DataGrip section to ru * Translate DataGrip section to zh * Translate DataGrip section to fa * Translate DBeaver section to fa * Translate DBeaver section to zh * Split third-party GUI to open-source and commercial * Mention some RDBMS integrations + ad-hoc translation fixes * Add rel="external nofollow" to outgoing links from docs * Lost blank lines * Fix class name * More rel="external nofollow" * Apply suggestions by @sundy-li * Mobile version of front page improvements * test * test 2 * test 3 * Update LICENSE * minor docs fix * Highlight current article as suggested by @sundy-li * fix link destination * Introduce backup.md (only "en" for now) * Mention INSERT+SELECT in backup.md * Some improvements for replication.md * Add backup.md to toc * Mention clickhouse-backup tool * Mention LightHouse in third-party GUI list * Introduce interfaces/third-party/proxy.md * Add clickhouse-bulk to proxy.md * Major extension of integrations.md contents * fix link target * remove unneeded file * better toc item name * fix markdown * better ru punctuation * Add yet another possible backup approach * Simplify copying permalinks to headers * Support non-eng link anchors in docs + update some deps * Generate anchors for single-page mode automatically * Remove anchors to top of pages * Remove anchors that nobody links to * build fixes * fix few links * restore css * fix some links * restore gifs * fix lost words * more docs fixes * docs fixes * NULL anchor * update urllib3 dependency * more fixes
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In order to store `NULL` in a table field, it must be of the [Nullable](../data_types/nullable.md) type.
Depending on the data format (input or output), `NULL` may have a different representation. For more information, see the documentation for [data formats](../interfaces/formats.md#formats).
There are many nuances to processing `NULL`. For example, if at least one of the arguments of a comparison operation is `NULL`, the result of this operation will also be `NULL`. The same is true for multiplication, addition, and other operations. For more information, read the documentation for each operation.
WIP on docs (#3813) * CLICKHOUSE-4063: less manual html @ index.md * CLICKHOUSE-4063: recommend markdown="1" in README.md * CLICKHOUSE-4003: manually purge custom.css for now * CLICKHOUSE-4064: expand <details> before any print (including to pdf) * CLICKHOUSE-3927: rearrange interfaces/formats.md a bit * CLICKHOUSE-3306: add few http headers * Remove copy-paste introduced in #3392 * Hopefully better chinese fonts #3392 * get rid of tabs @ custom.css * Apply comments and patch from #3384 * Add jdbc.md to ToC and some translation, though it still looks badly incomplete * minor punctuation * Add some backlinks to official website from mirrors that just blindly take markdown sources * Do not make fonts extra light * find . -name '*.md' -type f | xargs -I{} perl -pi -e 's//g' {} * find . -name '*.md' -type f | xargs -I{} perl -pi -e 's/ sql/g' {} * Remove outdated stuff from roadmap.md * Not so light font on front page too * Refactor Chinese formats.md to match recent changes in other languages * Update some links on front page * Remove some outdated comment * Add twitter link to front page * More front page links tuning * Add Amsterdam meetup link * Smaller font to avoid second line * Add Amsterdam link to README.md * Proper docs nav translation * Back to 300 font-weight except Chinese * fix docs build * Update Amsterdam link * remove symlinks * more zh punctuation * apply lost comment by @zhang2014 * Apply comments by @zhang2014 from #3417 * Remove Beijing link * rm incorrect symlink * restore content of docs/zh/operations/table_engines/index.md * CLICKHOUSE-3751: stem terms while searching docs * CLICKHOUSE-3751: use English stemmer in non-English docs too * CLICKHOUSE-4135 fix * Remove past meetup link * Add blog link to top nav * Add ContentSquare article link * Add form link to front page + refactor some texts * couple markup fixes * minor * Introduce basic ODBC driver page in docs * More verbose 3rd party libs disclaimer * Put third-party stuff into a separate folder * Separate third-party stuff in ToC too * Update links * Move stuff that is not really (only) a client library into a separate page * Add clickhouse-hdfs-loader link * Some introduction for "interfaces" section * Rewrite tcp.md * http_interface.md -> http.md * fix link * Remove unconvenient error for now * try to guess anchor instead of failing * remove symlink * Remove outdated info from introduction * remove ru roadmap.md * replace ru roadmap.md with symlink * Update roadmap.md * lost file * Title case in toc_en.yml * Sync "Functions" ToC section with en * Remove reference to pretty old ClickHouse release from docs * couple lost symlinks in fa * Close quote in proper place * Rewrite en/getting_started/index.md * Sync en<>ru getting_started/index.md * minor changes * Some gui.md refactoring * Translate DataGrip section to ru * Translate DataGrip section to zh * Translate DataGrip section to fa * Translate DBeaver section to fa * Translate DBeaver section to zh * Split third-party GUI to open-source and commercial * Mention some RDBMS integrations + ad-hoc translation fixes * Add rel="external nofollow" to outgoing links from docs * Lost blank lines * Fix class name * More rel="external nofollow" * Apply suggestions by @sundy-li * Mobile version of front page improvements * test * test 2 * test 3 * Update LICENSE * minor docs fix * Highlight current article as suggested by @sundy-li * fix link destination * Introduce backup.md (only "en" for now) * Mention INSERT+SELECT in backup.md * Some improvements for replication.md * Add backup.md to toc * Mention clickhouse-backup tool * Mention LightHouse in third-party GUI list * Introduce interfaces/third-party/proxy.md * Add clickhouse-bulk to proxy.md * Major extension of integrations.md contents * fix link target * remove unneeded file * better toc item name * fix markdown * better ru punctuation * Add yet another possible backup approach * Simplify copying permalinks to headers * Support non-eng link anchors in docs + update some deps * Generate anchors for single-page mode automatically * Remove anchors to top of pages * Remove anchors that nobody links to * build fixes * fix few links * restore css * fix some links * restore gifs * fix lost words * more docs fixes * docs fixes * NULL anchor * update urllib3 dependency * more fixes
2018-12-12 17:28:00 +00:00
In queries, you can check `NULL` using the [IS NULL](operators.md#operator-is-null) and [IS NOT NULL](operators.md) operators and the related functions `isNull` and `isNotNull`.
## Functions
Functions are written like an identifier with a list of arguments (possibly empty) in brackets. In contrast to standard SQL, the brackets are required, even for an empty arguments list. Example: `now()`.
There are regular and aggregate functions (see the section "Aggregate functions"). Some aggregate functions can contain two lists of arguments in brackets. Example: ` quantile (0.9) (x)`. These aggregate functions are called "parametric" functions, and the arguments in the first list are called "parameters". The syntax of aggregate functions without parameters is the same as for regular functions.
## Operators
Operators are converted to their corresponding functions during query parsing, taking their priority and associativity into account.
For example, the expression `1 + 2 * 3 + 4` is transformed to `plus(plus(1, multiply(2, 3)), 4)`.
## Data Types and Database Table Engines
Data types and table engines in the `CREATE` query are written the same way as identifiers or functions. In other words, they may or may not contain an arguments list in brackets. For more information, see the sections "Data types," "Table engines," and "CREATE".
## Expression Aliases {#syntax-expression_aliases}
Alias is a user defined name for an expression in a query.
```
expr AS alias
```
- `AS` — Keyword for defining aliases. You can define alias for a table name or a column name in the `SELECT` clause skipping `AS` keyword.
For example, `SELECT table_name_alias.column_name FROM table_name table_name_alias`.
WIP on docs (#3813) * CLICKHOUSE-4063: less manual html @ index.md * CLICKHOUSE-4063: recommend markdown="1" in README.md * CLICKHOUSE-4003: manually purge custom.css for now * CLICKHOUSE-4064: expand <details> before any print (including to pdf) * CLICKHOUSE-3927: rearrange interfaces/formats.md a bit * CLICKHOUSE-3306: add few http headers * Remove copy-paste introduced in #3392 * Hopefully better chinese fonts #3392 * get rid of tabs @ custom.css * Apply comments and patch from #3384 * Add jdbc.md to ToC and some translation, though it still looks badly incomplete * minor punctuation * Add some backlinks to official website from mirrors that just blindly take markdown sources * Do not make fonts extra light * find . -name '*.md' -type f | xargs -I{} perl -pi -e 's//g' {} * find . -name '*.md' -type f | xargs -I{} perl -pi -e 's/ sql/g' {} * Remove outdated stuff from roadmap.md * Not so light font on front page too * Refactor Chinese formats.md to match recent changes in other languages * Update some links on front page * Remove some outdated comment * Add twitter link to front page * More front page links tuning * Add Amsterdam meetup link * Smaller font to avoid second line * Add Amsterdam link to README.md * Proper docs nav translation * Back to 300 font-weight except Chinese * fix docs build * Update Amsterdam link * remove symlinks * more zh punctuation * apply lost comment by @zhang2014 * Apply comments by @zhang2014 from #3417 * Remove Beijing link * rm incorrect symlink * restore content of docs/zh/operations/table_engines/index.md * CLICKHOUSE-3751: stem terms while searching docs * CLICKHOUSE-3751: use English stemmer in non-English docs too * CLICKHOUSE-4135 fix * Remove past meetup link * Add blog link to top nav * Add ContentSquare article link * Add form link to front page + refactor some texts * couple markup fixes * minor * Introduce basic ODBC driver page in docs * More verbose 3rd party libs disclaimer * Put third-party stuff into a separate folder * Separate third-party stuff in ToC too * Update links * Move stuff that is not really (only) a client library into a separate page * Add clickhouse-hdfs-loader link * Some introduction for "interfaces" section * Rewrite tcp.md * http_interface.md -> http.md * fix link * Remove unconvenient error for now * try to guess anchor instead of failing * remove symlink * Remove outdated info from introduction * remove ru roadmap.md * replace ru roadmap.md with symlink * Update roadmap.md * lost file * Title case in toc_en.yml * Sync "Functions" ToC section with en * Remove reference to pretty old ClickHouse release from docs * couple lost symlinks in fa * Close quote in proper place * Rewrite en/getting_started/index.md * Sync en<>ru getting_started/index.md * minor changes * Some gui.md refactoring * Translate DataGrip section to ru * Translate DataGrip section to zh * Translate DataGrip section to fa * Translate DBeaver section to fa * Translate DBeaver section to zh * Split third-party GUI to open-source and commercial * Mention some RDBMS integrations + ad-hoc translation fixes * Add rel="external nofollow" to outgoing links from docs * Lost blank lines * Fix class name * More rel="external nofollow" * Apply suggestions by @sundy-li * Mobile version of front page improvements * test * test 2 * test 3 * Update LICENSE * minor docs fix * Highlight current article as suggested by @sundy-li * fix link destination * Introduce backup.md (only "en" for now) * Mention INSERT+SELECT in backup.md * Some improvements for replication.md * Add backup.md to toc * Mention clickhouse-backup tool * Mention LightHouse in third-party GUI list * Introduce interfaces/third-party/proxy.md * Add clickhouse-bulk to proxy.md * Major extension of integrations.md contents * fix link target * remove unneeded file * better toc item name * fix markdown * better ru punctuation * Add yet another possible backup approach * Simplify copying permalinks to headers * Support non-eng link anchors in docs + update some deps * Generate anchors for single-page mode automatically * Remove anchors to top of pages * Remove anchors that nobody links to * build fixes * fix few links * restore css * fix some links * restore gifs * fix lost words * more docs fixes * docs fixes * NULL anchor * update urllib3 dependency * more fixes
2018-12-12 17:28:00 +00:00
In the [CAST function](functions/type_conversion_functions.md), the `AS` keyword has another meaning. See the description of the function.
- `expr` — Any expression supported by ClickHouse.
For example `SELECT column_name * 2 AS double FROM some_table`.
- `alias` — Name for `expr`. Aliases should comply with the [identifiers](#syntax-identifiers) syntax.
For example, `SELECT "table t".column_name FROM table_name AS "table t"`.
### Peculiarities of Use
Aliases are global for a query or subquery and you can define alias in any part of a query for any expression. For example, `SELECT (1 AS n) + 2, n`.
Aliases are not visible in subqueries. For example, while executing the query `SELECT (SELECT sum(b.a) + num FROM b) - a.a AS num FROM a` ClickHouse generates the exception `Unknown identifier: num`.
If an alias is defined for result columns in `SELECT` clause in a subquery, these columns are visible in outer query. For example, `SELECT n + m FROM (SELECT 1 AS n, 2 AS m)`.
Be careful with aliases the same as column or table names. Let's consider the following example:
```
CREATE TABLE t
(
a Int,
b Int
)
ENGINE = TinyLog()
```
```
SELECT
argMax(a, b),
sum(b) AS b
FROM t
Received exception from server (version 18.14.17):
Code: 184. DB::Exception: Received from localhost:9000, 127.0.0.1. DB::Exception: Aggregate function sum(b) is found inside another aggregate function in query.
```
In this example, we declared table `t` with column `b`. Then, when selecting data, we defined the `sum(b) AS b` alias. As aliases are global, ClickHouse substituted the literal `b` in the expression `argMax(a, b)` with the expression `sum(b)`. This substitution caused the exception.
## Asterisk
In a `SELECT` query, an asterisk can replace the expression. For more information, see the section "SELECT".
## Expressions
An expression is a function, identifier, literal, application of an operator, expression in brackets, subquery, or asterisk. It can also contain an alias.
A list of expressions is one or more expressions separated by commas.
Functions and operators, in turn, can have expressions as arguments.
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[Original article](https://clickhouse.yandex/docs/en/query_language/syntax/) <!--hide-->