SQL Notes - Page 11

UPDATE: Conditional and Multi Column Changes

UPDATE changes existing rows. Learn multi column updates, conditional logic with CASE, updates driven by a join and how to check the blast radius firs...

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INSERT: Single Row, Multi Row and INSERT SELECT

Every way to put rows into a table: explicit column lists, multi row VALUES, INSERT ... SELECT, defaults and handling duplicate keys.

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DROP, TRUNCATE and DELETE Compared

Three ways to remove data, with very different consequences. Learn what each one destroys, what can be rolled back and what resets identity counters.

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ALTER TABLE: Adding, Modifying and Removing Columns

ALTER changes a table that already holds data. Add and drop columns, change types, set defaults and manage constraints without losing rows.

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CREATE: Databases, Schemas and Tables

CREATE defines the objects data lives in. Learn CREATE DATABASE, CREATE SCHEMA and CREATE TABLE, with column definitions and inline constraints.

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The Sample Database Used in This Path

Two small schemas - a company HR database and a shop sales database - used by every example in this SQL path. Create them once and follow along.

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SQL Operators and Expressions

Arithmetic, comparison, logical, string and set operators, how precedence works, and where an expression may legally appear.

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SQL Data Types Explained

Numeric, string, date and boolean types across the major dialects, and how to pick the type that stops bad data at the door.

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SQL Syntax, Keywords, Identifiers and Comments

How a SQL statement is built: clause order, keywords versus identifiers, quoting rules, statement terminators and the two comment styles.

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The Relational Model: Tables, Rows, Columns and Schemas

Tables hold typed columns and unordered rows, schemas hold tables, and every value obeys the type its column declares.

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What is SQL: Databases, DBMS and RDBMS

SQL is the declarative language used to define and query relational data. Start here to see what a database, a DBMS and an RDBMS actually are.