PostgreSQL中NULL陷阱的排除过滤指南

AND NOT (region = 'BR' AND order_status = 'CANCELED')

NULL = 'CANCELED'    → UNKNOWN
NULL != 'CANCELED'   → UNKNOWN
NULL AND TRUE        → UNKNOWN
NOT NULL             → UNKNOWN

AND NOT (region = 'BR' AND order_status = 'CANCELED')

AND (region IS NULL OR region != 'BR' OR order_status IS NULL OR order_status != 'CANCELED')

-- ❌ 错误：order_status 是 NULL 的行也会被过滤掉
WHERE order_status != 'CANCELED'

-- ✅ 正确：明确保留 NULL
WHERE order_status != 'CANCELED' OR order_status IS NULL

-- ❌ 危险：子查询结果中有一个 NULL，整个查询返回空！
WHERE order_id NOT IN (SELECT order_id FROM blacklist_orders)

-- ✅ 安全写法：过滤子查询中的 NULL
WHERE order_id NOT IN (
    SELECT order_id FROM blacklist_orders WHERE order_id IS NOT NULL
)

-- ✅ 更推荐：用 NOT EXISTS，天然不受 NULL 影响
WHERE NOT EXISTS (
    SELECT 1 FROM blacklist_orders WHERE blacklist_orders.order_id = t.order_id
)

COUNT(*)           -- 统计所有行，NULL 也计入
COUNT(order_status) -- 忽略 NULL 行，两者结果可能不同！

SUM(amount)        -- NULL 行被忽略，不是当 0 处理
AVG(amount)        -- 分母只统计非 NULL 行，结果可能偏高

-- 检查字段 NULL 数量
SELECT
    COUNT(*) AS total,
    COUNT(region) AS region_not_null,
    COUNT(*) - COUNT(region) AS region_null_count,
    COUNT(order_status) AS status_not_null,
    COUNT(*) - COUNT(order_status) AS status_null_count
FROM orders
WHERE geo = 'LA';

✅ 显式优于隐式 —— 意图要写清楚，不要靠"副作用"
✅ 先查 NULL 分布 —— 动手写条件前，先确认字段是否可空
✅ UNKNOWN ≠ FALSE —— NULL 参与运算结果是 UNKNOWN，WHERE 会过滤它

<!-- 只在 geo = LA 时追加此过滤条件 -->
<if test="geo == 'LA'">
    AND (region IS NULL OR region != 'BR'
         OR order_status IS NULL OR order_status != 'CANCELED')
</if>

 <if test="geo == 'LA'">
    and not (COALESCE(region_cd,'') = 'BR' AND COALESCE (order_status,'') = 'CREDIT NOTE')
</if>