系統講解 WHERE 子句常用操作符：比較、範圍、空值、邏輯組合，以及子句順序規則。
SELECT ... FROM ... WHERE ... ORDER BY ... LIMIT ... — WHERE 必須在 ORDER BY 之前。
| 類別 | 操作符 | 說明 | 示例 |
|---|---|---|---|
| 比較 | = | 等於 | WHERE vend_id = 1003 |
<> / != | 不等於 | WHERE vend_id <> 1003 | |
<, <=, >, >= | 大小比較 | WHERE prod_price < 10 | |
BETWEEN ... AND ... | 範圍（含邊界） | WHERE prod_price BETWEEN 5 AND 10 | |
IS NULL | 空值判斷 | WHERE prod_price IS NULL | |
IS NOT NULL | 非空判斷 | WHERE prod_price IS NOT NULL | |
| 邏輯 | AND | 邏輯與（優先級高） | WHERE vend_id = 1003 AND prod_price < 10 |
OR | 邏輯或 | WHERE vend_id = 1003 OR vend_id = 1004 | |
NOT | 邏輯非 | WHERE NOT vend_id = 1003 |
NULL 陷阱： NULL != 任何值（包括 NULL），只能用 IS NULL / IS NOT NULL。AND 優先於 OR，複雜條件必須用括號明確意圖：WHERE (a=1 OR a=2) AND b=3。BETWEEN 含邊界： BETWEEN 5 AND 10 等價 >=5 AND <=10。
