MSDN Documentation

1. Parsing

The parsing phase is the first step where the database engine analyzes the syntactical structure of the incoming SQL query. This involves:

• Lexical Analysis (Tokenization): The query string is broken down into individual units called tokens. These tokens represent keywords (e.g., SELECT, FROM, WHERE), identifiers (table names, column names), operators (=, >), literals (numbers, strings), and punctuation ((, ), ,).
• Syntactic Analysis: The sequence of tokens is checked against the defined grammar rules of the SQL language. If the query violates these rules, a syntax error is reported, and the query execution is halted. This phase effectively builds an internal representation of the query, often in the form of a parse tree or abstract syntax tree (AST).

For example, a simple query like:

SELECT CustomerName FROM Customers WHERE Country = 'USA';

Would be tokenized into:

• SELECT (Keyword)
• CustomerName (Identifier)
• FROM (Keyword)
• Customers (Identifier)
• WHERE (Keyword)
• Country (Identifier)
• = (Operator)
• 'USA' (Literal)
• ; (Punctuation)

The parser then verifies if this sequence adheres to the SQL language's structure for a SELECT statement.

2. Binding (Semantic Analysis)

Once the query is syntactically validated, the binding phase, also known as semantic analysis, takes place. This stage resolves references to database objects and ensures the query is meaningful in the context of the database schema. Key aspects include:

• Object Resolution: The engine looks up all referenced tables, views, columns, functions, and other database objects in the system catalog. It verifies their existence and the user's permissions to access them.
• Data Type Checking: It checks for compatibility between data types involved in operations (e.g., comparing a string to a number, or applying a function to an inappropriate data type). Implicit conversions might be performed where possible and appropriate.
• Ambiguity Resolution: If a column name is ambiguous (e.g., exists in multiple tables in the FROM clause), the engine requires explicit qualification (e.g., TableName.ColumnName) or uses context to resolve it.
• Subquery Correlation: For subqueries, the binding process establishes the correlation between outer and inner queries, if applicable.

If any semantic issues are found (e.g., a table or column doesn't exist, or a data type mismatch cannot be resolved), an error is raised, and the query fails. A successful binding process transforms the parse tree into a richer representation, often called a query block or a logical query plan, which is then passed to the query optimizer.

Importance of Parsing and Binding

These initial stages are crucial because they:

• Ensure the validity and correctness of the SQL statement before attempting execution.
• Identify potential errors early, saving resources that would otherwise be spent on invalid queries.
• Lay the groundwork for subsequent query optimization and execution phases by providing a semantically understood representation of the query.
• Determine the scope and context of all identifiers and expressions within the query.