This section covers the major lexical structure of SQL, which for the most part, is going to resemble that of ANSI SQL itself hence why low-levels details are not discussed in depth.
{es-sql} currently accepts only one _command_ at a time. A command is a sequence of _tokens_ terminated by the end of input stream.
A token can be a __key word__, an _identifier_ (_quoted_ or _unquoted_), a _literal_ (or constant) or a special character symbol (typically a delimiter). Tokens are typically separated by whitespace (be it space, tab) though in some cases, where there is no ambiguity (typically due to a character symbol) this is not needed - however for readability purposes this should be avoided.
This query has four tokens: `SELECT`, `*`, `FROM` and `table`. The first three, namely `SELECT`, `*` and `FROM` are __key words__ meaning words that have a fixed meaning in SQL. The token `table` is an _identifier_ meaning it identifies (by name) an entity inside SQL such as a table (in this case), a column, etc...
As one can see, both key words and identifiers have the _same_ lexical structure and thus one cannot know whether a token is one or the other without knowing the SQL language; the complete list of key words is available in the <<sql-syntax-reserved, reserved appendix>>.
Do note that key words are case-insensitive meaning the previous example can be written as:
[source, sql]
----
select * fRoM table;
----
Identifiers however are not - as {es} is case sensitive, {es-sql} uses the received value verbatim.
To help differentiate between the two, through-out the documentation the SQL key words are upper-cased a convention we find increases readability and thus recommend to others.
[[sql-syntax-identifiers]]
[float]
=== Identifiers
Identifiers can be of two types: __quoted__ and __unquoted__:
This query has two identifiers, `ip_address` and `hosts-*` (an <<multi-index,index pattern>>). As `ip_address` does not clash with any key words it can be used verbatim, `hosts-*` on the other hand cannot as it clashes with `-` (minus operation) and `*` hence the double quotes.
The first identifier from needs to quoted as otherwise it clashes with the `FROM` key word (which is case insensitive as thus can be written as `from`) while the second identifier using {es} <<date-math-index-names>> would have otherwise confuse the parser.
Hence why in general, *especially* when dealing with user input it is *highly* recommended to use quotes for identifiers. It adds minimal increase to your queries and in return offers clarity and disambiguation.
[[sql-syntax-literals]]
[float]
=== Literals (Constants)
{es-sql} supports two kind of __implicitly-typed__ literals: strings and numbers.
[[sql-syntax-string-literals]]
[float]
==== String Literals
A string literal is an arbitrary number of characters bounded by single quotes `'`: `'Giant Robot'`.
To include a single quote in the string, escape it using another single quote: `'Captain EO''s Voyage'`.
NOTE: An escaped single quote is *not* a double quote (`"`), but a single quote `'` _repeated_ (`''`).
[sql-syntax-numeric-literals]
[float]
==== Numeric Literals
Numeric literals are accepted both in decimal and scientific notation with exponent marker (`e` or `E`), starting either with a digit or decimal point `.`:
[source, sql]
----
1969 -- integer notation
3.14 -- decimal notation
.1234 -- decimal notation starting with decimal point
4E5 -- scientific notation (with exponent marker)
1.2e-3 -- scientific notation with decimal point
----
Numeric literals that contain a decimal point are always interpreted as being of type `double`. Those without are considered `integer` if they fit otherwise their type is `long` (or `BIGINT` in ANSI SQL types).
[[sql-syntax-generic-literals]]
[float]
==== Generic Literals
When dealing with arbitrary type literal, one creates the object by casting, typically, the string representation to the desired type. This can be achieved through the dedicated <<sql-operators-cast, cast operator>> and <<sql-functions-type-conversion, functions>>:
[source, sql]
----
123::LONG -- cast 123 to a LONG
CAST('1969-05-13T12:34:56' AS TIMESTAMP) -- cast the given string to datetime
CONVERT('10.0.0.1', IP) -- cast '10.0.0.1' to an IP
----
Do note that {es-sql} provides functions that out of the box return popular literals (like `E()`) or provide dedicated parsing for certain strings.
[[sql-syntax-single-vs-double-quotes]]
[float]
=== Single vs Double Quotes
It is worth pointing out that in SQL, single quotes `'` and double quotes `"` have different meaning and *cannot* be used interchangeably.
Single quotes are used to declare a <<sql-syntax-string-literals, string literal>> while double quotes for <<sql-syntax-identifiers, identifiers>>.
To wit:
[source, sql]
----
SELECT "first_name" <1>
FROM "musicians" <1>
WHERE "last_name" <1>
= 'Carroll' <2>
----
<1> Double quotes `"` used for column and table identifiers
<2> Single quotes `'` used for a string literal
[[sql-syntax-special-chars]]
[float]
=== Special characters
A few characters that are not alphanumeric have a dedicated meaning different from that of an operator. For completeness these are specified below:
[cols="^m,^15"]
|===
s|Char
s|Description
|* | The asterisk (or wildcard) is used in some contexts to denote all fields for a table. Can be also used as an argument to some aggregate functions.
|, | Commas are used to enumerate the elements of a list.
|. | Used in numeric constants or to separate identifiers qualifiers (catalog, table, column names, etc...).
|()| Parentheses are used for specific SQL commands, function declarations or to enforce precedence.
|===
[[sql-syntax-operators]]
[float]
=== Operators
Most operators in {es-sql} have the same precedence and are left-associative. As this is done at parsing time, parenthesis need to be used to enforce a different precedence.
The following table indicates the supported operators and their precendence (highest to lowest);
[cols="^2m,^,^3"]
|===
s|Operator/Element
s|Associativity
s|Description
|.
|left
|qualifier separator
|::
|left
|PostgreSQL-style type cast
|+ -
|right
|unary plus and minus (numeric literal sign)
|* / %
|left
|multiplication, division, modulo
|+ -
|left
|addition, substraction
|BETWEEN IN LIKE
|
|range containment, string matching
|< > <= >= = <=> <> !=
|
|comparison
|NOT
|right
|logical negation
|AND
|left
|logical conjunction
|OR
|left
|logical disjunction
|===
[[sql-syntax-comments]]
[float]
=== Comments
{es-sql} allows comments which are sequence of characters ignored by the parsers.
Two styles are supported:
Single Line:: Comments start with a double dash `--` and continue until the end of the line.
