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.
INSERT query uses both parsers:
INSERT INTO t VALUES (1, 'Hello, world'), (2, 'abc'), (3, 'def')
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. You can also turn on the full parser for the data by using the input_format_values_interpret_expressions setting. When
input_format_values_interpret_expressions = 1, ClickHouse first tries to parse values with the fast stream parser. If it fails, ClickHouse tries to use the full parser for the data, treating it like an SQL expression.
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.
It allows for avoiding issues with large
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.
The rest of this article covers the full parser. For more information about format parsers, see the Formats section.
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.
ClickHouse supports either SQL-style and C-style comments:
- SQL-style comments start with
#and continue to the end of the line, a space after
#!can be omitted.
- C-style are from
*/and can be multiline, spaces are not required either.
Keywords are case-insensitive when they correspond to:
- SQL standard. For example,
SeLeCtare all valid.
- Implementation in some popular DBMS (MySQL or Postgres). For example,
DateTimeis the same as
You can check whether a data type name is case-sensitive in the system.data_type_families table.
In contrast to standard SQL, all other keywords (including functions names) are case-sensitive.
Keywords are not reserved; they are treated as such only in the corresponding context. If you use identifiers with the same name as the keywords, enclose them into double-quotes or backticks. For example, the query
SELECT "FROM" FROM table_name is valid if the table
table_name has column with the name
- Cluster, database, table, partition, and column names.
- Data types.
- Expression aliases.
Identifiers can be quoted or non-quoted. The latter is preferred.
Non-quoted identifiers must match the regex
^[a-zA-Z_][0-9a-zA-Z_]*$ and can not be equal to keywords. Examples:
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,
There are numeric, string, compound, and
Numeric literal tries to be parsed:
- First, as a 64-bit signed number, using the strtoull function.
- If unsuccessful, as a 64-bit unsigned number, using the strtoll function.
- If unsuccessful, as a floating-point number using the strtod function.
- Otherwise, it returns an error.
Literal value has 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.
Only string literals in single quotes are supported. The enclosed characters can be backslash-escaped. The following escape sequences have a corresponding special value:
\xHH. In all other cases, escape sequences in the format
c is any character, are converted to
c. It means that you can use the sequences
\\. The value will have the String type.
In string literals, you need to escape at least
\. Single quotes can be escaped with the single quote, literals
'It''s' are equal.
Arrays are constructed with square brackets
[1, 2, 3]. Tuples are constructed with round brackets
(1, 'Hello, world!', 2).
Technically 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.
There’s a separate case when tuples appear in the
IN clause of a
SELECT query. Query results can include tuples, but tuples can’t be saved to a database (except of tables with Memory engine).
Indicates that the value is missing.
In order to store
NULL in a table field, it must be of the Nullable type.
Depending on the data format (input or output),
NULL may have a different representation. For more information, see the documentation for data 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 is also
NULL. The same is true for multiplication, addition, and other operations. For more information, read the documentation for each operation.
A heredoc is a way to define a string (often multiline), while maintaining the original formatting. A heredoc is defined as a custom string literal, placed between two
$ symbols, for example
$heredoc$. A value between two heredocs is processed "as-is".
You can use a heredoc to embed snippets of SQL, HTML, or XML code, etc.
SELECT $smth$SHOW CREATE VIEW my_view$smth$;
┌─'SHOW CREATE VIEW my_view'─┐
│ SHOW CREATE VIEW my_view │
Function calls are written like an identifier with a list of arguments (possibly empty) in round brackets. In contrast to standard SQL, the brackets are required, even for an empty argument list. Example:
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 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 argument list in brackets. For more information, see the sections “Data types,” “Table engines,” and “CREATE”.
An alias is a user-defined name for expression in a query.
expr AS alias
AS— The keyword for defining aliases. You can define the alias for a table name or a column name in a
SELECTclause without using the
For example, `SELECT table_name_alias.column_name FROM table_name table_name_alias`.
In the [CAST](/docs/en/sql-reference/functions/type-conversion-functions#type_conversion_function-cast) function, 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.
For example, `SELECT "table t".column_name FROM table_name AS "table t"`.
Notes on Usage
Aliases are global for a query or subquery, and you can define an alias in any part of a query for any expression. For example,
SELECT (1 AS n) + 2, n.
Aliases are not visible in subqueries and between 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 the result columns in the
SELECT clause of a subquery, these columns are visible in the outer query. For example,
SELECT n + m FROM (SELECT 1 AS n, 2 AS m).
Be careful with aliases that are the same as column or table names. Let’s consider the following example:
CREATE TABLE t
ENGINE = TinyLog()
sum(b) AS b
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. You can change this default behavior by setting prefer_column_name_to_alias to
SELECT query, an asterisk can replace the expression. For more information, see the section “SELECT”.
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.