What is SOQL and how does it differ from SQL?

SOQL (Salesforce Object Query Language) is Salesforce's query language for retrieving records. Inspired by SQL but with key differences:

• No `SELECT *` — you must list every field. Fields not selected return null on the resulting sObject.
• Single-object queries with relationship traversal — SELECT Id, Account.Name FROM Contact traverses the lookup, but classic SQL JOIN syntax doesn't exist. You use dot notation for parent and subqueries for children: SELECT Id, (SELECT Id FROM Contacts) FROM Account.
• Limited operators — no UNION, no HAVING (use GROUP BY HAVING only), no full-text via LIKE (use SOSL for that).
• Governor-limit aware — every query counts against the per-transaction SOQL limit (100 sync, 200 async).
• Index-driven — for large tables, queries must hit indexed fields to be selective.
• Polymorphic queries — query on Lookup-to-Polymorphic relationships (SELECT Id, What.Type FROM Task).

Use SOSL instead of SOQL for full-text search across multiple objects.