Builder Pattern: SQL Query Builder

Problem

Building a SQL query string by concatenating strings leads to bugs and unreadable code. Design a QueryBuilder using the Builder pattern that lets you construct SELECT queries step-by-step with a fluent interface, then call .build() to get the final SQL string.

Requirements

QueryBuilder.select(...columns) — which columns to fetch
QueryBuilder.from(table) — source table
QueryBuilder.where(condition) — optional filter (multiple calls = AND)
QueryBuilder.orderBy(column, direction) — optional sort
QueryBuilder.limit(n) — optional row limit
QueryBuilder.build() — returns the final SQL string
Method chaining (fluent interface): each method returns `this`

Constraints

–from() is mandatory — build() throws if no table set
–select() defaults to '*' if not called
–Multiple where() calls are joined with AND

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