#MySQL NULL值处理

-- Check NULL values

-- Find rows with NULL values

-- Find rows without NULL values

-- Multiple NULL checks

-- Return first non-NULL value

-- Nested COALESCE

-- Return second value if first is NULL

-- In calculations

-- These comparisons return NULL, not TRUE or FALSE

-- Use IS NULL instead

-- Correct: Use IS NULL

-- Incorrect: Will not work as expected

-- Multiple NULL conditions

-- Using OR with NULL

-- Any operation with NULL returns NULL
-- If discount is NULL, total is NULL

-- Use IFNULL or COALESCE to handle

-- Concatenation with NULL
-- If middle_name is NULL, full_with_middle is NULL

-- Use CONCAT_WS (skip NULL)
-- CONCAT_WS skips NULL values

-- Return NULL if two expressions are equal
-- Returns NULL if price equals discount_price

-- Use in calculations

-- COUNT(*) includes NULL
-- COUNT(column) excludes NULL

-- Aggregate functions ignore NULL
-- SUM and AVG only include non-NULL values

-- With NULL handling

-- NULL values are grouped together
-- All NULL departments are in one group

-- Handle NULL in grouping

-- NULL values in sorting

-- Control NULL placement (MySQL 8.0+)

-- Custom NULL ordering (older versions)

-- Multiple NULL values allowed in UNIQUE column

-- These are allowed
-- Multiple NULL emails are allowed

-- But duplicate non-NULL values are not
-- Error: Duplicate entry

-- NULL allowed in foreign key

-- Orders without customer (NULL customer_id allowed)

-- Find orders without customer

-- Find orders with customer

-- Test if expression is NULL

-- NVL equivalent to IFNULL

-- LEAST and GREATEST return NULL if any argument is NULL

-- Use COALESCE to handle NULL

-- Views can contain NULL values

-- Query view

    -- Set default if NULL
    

-- Call procedure

-- Use NULL for optional data

-- Use DEFAULT values instead of NULL when appropriate

-- Index columns with NULL
-- NULL values can affect index usage

-- Use IS NOT NULL for indexed columns

-- Use NOT NULL for required fields