#MySQL分组

-- Group by single column

-- Group by country

-- Count per category

-- Group by multiple columns

-- Detailed grouping

-- Count all rows

-- Count non-NULL values

-- Count distinct values

-- Count per group

-- Sum of values

-- Sum per group

-- Sum with condition

-- Average value

-- Average per group

-- Round average

-- Minimum value

-- Maximum value

-- First and last date

-- Filter groups with HAVING

-- Having with aggregate

-- WHERE filters rows before grouping

-- HAVING filters groups after grouping

-- Multiple conditions

-- With OR

-- Group with JOIN

-- Complex JOIN with GROUP BY

-- Generate subtotals and grand total

-- Result includes:
-- - Details by category and subcategory
-- - Subtotals by category
-- - Grand total

-- Identify NULL from ROLLUP

-- 1 = generated by ROLLUP (supertotal row)
-- 0 = normal grouping row

-- Generate all combinations

-- Creates summaries for:
-- - All combinations of category and subcategory
-- - Each category total
-- - Each subcategory total
-- - Grand total

-- Specific groupings

-- ROLLUP equivalent
-- Equivalent to:

-- CUBE equivalent
-- Equivalent to:

-- Count distinct values per group

-- Complex distinct count

-- Daily sales summary

-- Users by activity level

-- Products by category with stats

-- Index on GROUP BY columns helps

-- Index for WHERE + GROUP BY

-- Filter before grouping

-- Use appropriate data types
-- Numeric grouping is faster than string grouping

-- Avoid SELECT * with GROUP BY
-- Select only necessary columns

-- Column not in GROUP BY or aggregate
-- Error: Expression #X of SELECT list is not in GROUP BY
-- Solution: Add column to GROUP BY or use aggregate

-- Unexpected results
-- Check: GROUP BY columns, aggregate functions
-- Verify: Filter conditions in WHERE vs HAVING

-- Check group formation

-- Verify aggregate values