Provides an SQL parser and object model. This package, and the dependent `org.eigenbase.sql.parser` package, are independent of the other saffron packages, so may be used standalone.

Revision | $Id: //open/dev/farrago/src/org/eigenbase/sql/package.html#7 $
Copyright | Copyright (C) 2005-2009 The Eigenbase Project Copyright (C) 2003-2009 SQLstream, Inc. Copyright (C) 2005-2009 LucidEra, Inc.
Author | Julian Hyde

## Parser

{@link org.eigenbase.sql.parser.SqlParser} parses a SQL string to a parse tree. It only performs the most basic syntactic validation.

## Object model

Every node in the parse tree is a {@link org.eigenbase.sql.SqlNode}. Sub-types are:

- {@link org.eigenbase.sql.SqlLiteral} represents a boolean, numeric, string, or date constant, or the value `NULL`.
- {@link org.eigenbase.sql.SqlIdentifier} represents an identifier, such as `EMPNO` or `emp.deptno`.
- {@link org.eigenbase.sql.SqlCall} is a call to an operator or function. By means of special operators, we can use this construct to represent virtually every non-leaf node in the tree. For example, a `select` statement is a call to the 'select' operator.
- {@link org.eigenbase.sql.SqlNodeList} is a list of nodes.

A {@link org.eigenbase.sql.SqlOperator} describes the behavior of a node in the tree, such as how to un-parse a {@link org.eigenbase.sql.SqlCall} into a SQL string. It is important to note that operators are metadata, not data: there is only one `SqlOperator` instance representing the '=' operator, even though there may be many calls to it.

`SqlOperator` has several derived classes which make it easy to define new operators: {@link org.eigenbase.sql.SqlFunction}, {@link org.eigenbase.sql.SqlBinaryOperator}, {@link org.eigenbase.sql.SqlPrefixOperator}, {@link org.eigenbase.sql.SqlPostfixOperator}. And there are singleton classes for special syntactic constructs {@link org.eigenbase.sql.SqlSelectOperator} and {@link org.eigenbase.sql.SqlJoinOperator}. (These special operators even have their own sub-types of {@link org.eigenbase.sql.SqlCall}: {@link org.eigenbase.sql.SqlSelect} and {@link org.eigenbase.sql.SqlJoin}.)

A {@link org.eigenbase.sql.SqlOperatorTable} is a collection of operators. By supplying your own operator table, you can customize the dialect of SQL without modifying the parser.

## Validation

{@link org.eigenbase.sql.validate.SqlValidator} checks that a tree of {@link org.eigenbase.sql.SqlNode}s is semantically valid. You supply a {@link org.eigenbase.sql.SqlOperatorTable} to describe the available functions and operators, and a {@link org.eigenbase.sql.validate.SqlValidatorCatalogReader} for access to the database's catalog.

## Generating SQL

A {@link org.eigenbase.sql.SqlWriter} converts a tree of {@link org.eigenbase.sql.SqlNode}s into a SQL string. A {@link org.eigenbase.sql.SqlDialect} defines how this happens.