#MySQL联合查询

-- Removes duplicates

-- Each country appears only once

-- Keeps all rows including duplicates

-- All countries from both tables

-- Combine results from two tables

-- Different tables, same structure

-- Must have same number of columns

-- Compatible data types

-- Add conditions to each SELECT

-- Complex conditions

-- Sort entire result set

-- Sort individual SELECTs

-- Top from each source

-- Limited union result

-- Three-table union

-- Four-table union

-- Union with subqueries

-- Complex subquery union

-- Find all unique email addresses

-- Indexes on WHERE columns improve performance

-- Ensure indexes on join/where columns

-- Use UNION ALL when possible

-- Limit results early