QueryLanguage   = SelectStatement | UpdateStatement | DeleteStatement

SelectStatement = [SelectClause] FromClause [WhereClause] [GroupByClause] [HavingClause] [OrderByClause] [LimitClause]
UpdateStatement = UpdateClause [WhereClause] [OrderByClause] [LimitClause]
DeleteStatement = DeleteClause [WhereClause] [OrderByClause] [LimitClause]

Subselect       = SimpleSelectClause FromClause [WhereClause] [GroupByClause] [HavingClause] [OrderByClause] [LimitClause] [OffsetClause]
SelectClause    = "SELECT" ["ALL" | "DISTINCT"] SelectExpression {"," SelectExpression}
SimpleSelectClause = "SELECT" ["ALL" | "DISTINCT"] SelectExpression
DeleteClause    = "DELETE" "FROM" RangeVariableDeclaration
WhereClause     = "WHERE" ConditionalExpression
FromClause      = "FROM" IdentificationVariableDeclaration {"," IdentificationVariableDeclaration}
HavingClause    = "HAVING" ConditionalExpression
GroupByClause   = "GROUP" "BY" GroupByItem {"," GroupByItem}
OrderByClause   = "ORDER" "BY" OrderByItem {"," OrderByItem}
LimitClause     = "LIMIT" Expression ["OFFSET" Expression]
UpdateClause    = "UPDATE" RangeVariableDeclaration "SET" UpdateItem {"," UpdateItem}

OrderByItem = PathExpression ["ASC" | "DESC"]
GroupByItem = PathExpression
UpdateItem = PathExpression "=" (Expression | "NULL")

IdentificationVariableDeclaration =  RangeVariableDeclaration {Join}
RangeVariableDeclaration = PathExpression [["AS" ] identifier<identification-variable>]

Join = ["LEFT" | "INNER"] "JOIN" PathExpression "AS" identifier [["ON" | "WITH"] ConditionalExpression] IndexBy
IndexBy = "INDEXBY" identifier

ConditionalExpression = ConditionalTerm {"OR" ConditionalTerm}
ConditionalTerm       = ConditionalFactor {"AND" ConditionalFactor}
ConditionalFactor     = ["NOT"] ConditionalPrimary
ConditionalPrimary    = SimpleConditionalExpression | "(" ConditionalExpression ")"
SimpleConditionalExpression
                      = Expression (ComparisonExpression | BetweenExpression | LikeExpression
                      | InExpression | NullComparisonExpression) | ExistsExpression

Atom = string-literal | numeric-constant | input-parameter

Expression  = Expression {("+" | "-" | "*" | "/") Expression}
Expression  = ("+" | "-") Expression
Expression  = "(" Expression ")"
Expression  = PathExpression | Atom | | Function | AggregateExpression

SelectExpression  = (Expression | "(" Subselect ")" ) [["AS"] identifier]
PathExpression = identifier {"." identifier}

AggregateExpression = ("AVG" | "MAX" | "MIN" | "SUM") "(" ["DISTINCT"] Expression ")"
                    | "COUNT" "(" ["DISTINCT"] (Expression | "*") ")"

QuantifiedExpression     = ("ALL" | "ANY" | "SOME") "(" Subselect ")"
BetweenExpression        = ["NOT"] "BETWEEN" Expression "AND" Expression
ComparisonExpression     = ComparisonOperator ( QuantifiedExpression | Expression | "(" Subselect ")" )
InExpression             = ["NOT"] "IN" "(" (Atom {"," Atom} | Subselect) ")"
LikeExpression           = ["NOT"] "LIKE" Expression ["ESCAPE" escape_character]
NullComparisonExpression = "IS" ["NOT"] "NULL"
ExistsExpression         = ["NOT"] "EXISTS" "(" Subselect ")"


Function =
    "CURRENT_DATE" |
    "CURRENT_TIME" |
    "CURRENT_TIMESTAMP" |
    "LENGTH" "(" Expression ")" |
    "LOCATE" "(" Expression "," Expression ["," Expression] ")" |
    "ABS" "(" Expression ")" |
    "SQRT" "(" Expression ")" |
    "MOD" "(" Expression "," Expression ")" |
    "SIZE" "(" Expression ")" |
    "CONCAT" "(" Expression "," Expression ")" |
    "SUBSTRING" "(" Expression "," Expression "," "Expression" ")" |
    "TRIM" "(" [[TrimSpecification] [trim_character] "FROM"] string_primary ")" |
    "LOWER" "(" string_primary ")" |
    "UPPER" "(" string_primary ")" |
    identifier "(" [Expression {"," Expression}]")" // Custom function

TrimSpecification = "LEADING" | "TRAILING" | "BOTH"