Part 5: SQL Parsing: Turning Strings Into Commands

In Parts 1-4, we built a transactional key-value store. It has WAL for durability, memtables and SSTables for storage, compaction to control file growth, and transactions for atomic multi-key writes. Now we move to the query layer where users can actually fire SQL queries like: CREATE TABLE payments ( amount INT , id STRING , status STRING , captured BOOL , PRIMARY KEY ( id )) INSERT INTO payments VALUES ( 500 , payment_1 , pending , 1 ) SELECT * FROM payments WHERE id = payment_1 In this blog and the next one we answer: How do we translate SQL strings into operations our key-value store already understands? This post focuses on the first half of that bridge, which is parsing SQL into structured commands. In the next post, we will take those commands and turn CREATE TABLE and INSERT into bytes on disk. The Core Idea: SQL Becomes Structured Data The storage engine does not understand SQL. It understands keys, values, WAL entries, memtables, SSTables, and transactions. So the SQL layer has two jobs: Parse a human-readable SQL string into a structured object. Translate that structured object into key-value operations. For example: CREATE TABLE payments (...) -> CreateTable{TableName: "payments", ColumnDetails: ...} INSERT INTO payments VALUES (...) -> InsertIntoTable{TableName: "payments", ColumnValues: ...} SELECT * FROM payments WHERE id = payment_1 -> SelectFromTable{TableName: "payments", QueryConditions: ...} Once we have these structs, the rest of the database can stop dealing with raw strings. Why Not Parse SQL Directly in the DB Layer? Imagine if db.CreateTable() directly walked through the SQL string and also updated storage. That would mix two very different responsibilities: parsing grammar, executing database operations. Keeping them separate makes the system easier to reason about. The parser validates syntax and builds an AST. The DB layer receives that AST and decides what to store. An AST, or Abstract Syntax Tree, is just a structured representation of