{"id":2356,"date":"2025-03-04T20:13:06","date_gmt":"2025-03-04T12:13:06","guid":{"rendered":"https:\/\/www.laixuexila.com\/?p=2356"},"modified":"2025-03-04T20:13:06","modified_gmt":"2025-03-04T12:13:06","slug":"sqlite-null-%e5%80%bc","status":"publish","type":"post","link":"https:\/\/www.laixuexila.com\/index.php\/2025\/03\/04\/sqlite-null-%e5%80%bc\/","title":{"rendered":"SQLite\u00a0NULL \u503c"},"content":{"rendered":"\n
\n

\u5728 SQLite<\/strong> \u4e2d\uff0cNULL<\/code> \u662f\u4e00\u79cd\u7279\u6b8a\u503c\uff0c\u8868\u793a “\u672a\u77e5” \u6216 “\u4e0d\u5b58\u5728”<\/strong>\u3002\u5b83\u4e0e 0<\/code>\u3001''<\/code>\uff08\u7a7a\u5b57\u7b26\u4e32\uff09\u6216 FALSE<\/code> \u5e76\u4e0d\u76f8\u540c\u3002SQLite \u63d0\u4f9b\u4e86\u4e00\u4e9b\u64cd\u4f5c\u548c\u51fd\u6570\u6765\u5904\u7406 NULL<\/code> \u503c\u3002<\/p>\n<\/blockquote>\n\n\n\n

\n\n\n\n

1. NULL<\/code> \u503c\u7684\u7279\u70b9<\/strong><\/h2>\n\n\n\n
    \n
  • NULL<\/code> \u4e0d\u662f 0<\/strong> \u6216 ''<\/code>\uff08\u7a7a\u5b57\u7b26\u4e32\uff09\u3002<\/li>\n\n\n\n
  • \u4efb\u4f55 \u6d89\u53ca NULL<\/code> \u7684\u8ba1\u7b97\u7ed3\u679c\u90fd\u662f NULL<\/code><\/strong>\uff0c\u6bd4\u5982\uff1a<\/li>\n<\/ul>\n\n\n\n
      SELECT NULL + 10;  -- \u7ed3\u679c\u662f NULL\n  SELECT NULL || 'text'; -- \u7ed3\u679c\u662f NULL<\/code><\/pre>\n\n\n\n
      \n
    • NULL<\/code> \u5728\u6bd4\u8f83\u65f6\u5177\u6709\u7279\u6b8a\u884c\u4e3a\uff0c\u4f8b\u5982\uff1a<\/li>\n<\/ul>\n\n\n\n
        SELECT NULL = NULL;   -- \u7ed3\u679c\u662f NULL\uff08\u4e0d\u662f TRUE \u6216 FALSE\uff09\n  SELECT NULL <> NULL;  -- \u7ed3\u679c\u662f NULL<\/code><\/pre>\n\n\n\n
      \n\n\n\n
      2. NULL<\/code> \u503c\u7684\u63d2\u5165<\/strong><\/h2>\n\n\n\n
      \u793a\u4f8b\uff1a\u521b\u5efa\u8868\u5e76\u63d2\u5165 NULL<\/code><\/strong><\/h3>\n\n\n\n
      CREATE TABLE users (\n    id INTEGER PRIMARY KEY,\n    name TEXT,\n    age INTEGER\n);\n\nINSERT INTO users (id, name, age) VALUES (1, 'Alice', 25);\nINSERT INTO users (id, name, age) VALUES (2, 'Bob', NULL);\nINSERT INTO users (id, name, age) VALUES (3, NULL, 30);<\/code><\/pre>\n\n\n\n
      \u8fd9\u91cc\uff1a<\/p>\n\n\n\n
        \n
      • Bob<\/code> \u7684 age<\/code> \u662f NULL<\/code>\u3002<\/li>\n\n\n\n
      • \u7b2c\u4e09\u884c<\/code> \u7684 name<\/code> \u662f NULL<\/code>\u3002<\/li>\n<\/ul>\n\n\n\n
        \n\n\n\n
        3. NULL<\/code> \u7684\u6bd4\u8f83<\/strong><\/h2>\n\n\n\n
        (1) \u4e0d\u80fd\u7528 =<\/code> \u76f4\u63a5\u5224\u65ad NULL<\/code><\/strong><\/h3>\n\n\n\n
        \u9519\u8bef\u793a\u4f8b\uff1a<\/p>\n\n\n\n
        SELECT * FROM users WHERE age = NULL;<\/code><\/pre>\n\n\n\n
        \u8fd9\u4e2a\u67e5\u8be2\u4e0d\u4f1a\u8fd4\u56de\u4efb\u4f55\u7ed3\u679c\uff0c\u56e0\u4e3a NULL = NULL<\/code> \u4e0d\u662f TRUE<\/code>\uff0c\u800c\u662f NULL<\/code>\u3002<\/p>\n\n\n\n
        (2) IS NULL<\/code> & IS NOT NULL<\/code><\/strong><\/h3>\n\n\n\n
        \u6b63\u786e\u7684\u5224\u65ad\u65b9\u5f0f\uff1a<\/p>\n\n\n\n
        SELECT * FROM users WHERE age IS NULL;      -- \u67e5\u8be2 age \u4e3a NULL \u7684\u884c\nSELECT * FROM users WHERE name IS NOT NULL; -- \u67e5\u8be2 name \u4e0d\u4e3a NULL \u7684\u884c<\/code><\/pre>\n\n\n\n
        \n\n\n\n
        4. \u5904\u7406 NULL<\/code> \u7684\u51fd\u6570<\/strong><\/h2>\n\n\n\n
        SQLite \u63d0\u4f9b\u4e86\u4e00\u4e9b\u51fd\u6570\u6765\u5904\u7406 NULL<\/code> \u503c\u3002<\/p>\n\n\n\n
        (1) IFNULL()<\/code>\uff1a\u5982\u679c\u662f NULL<\/code>\uff0c\u5219\u8fd4\u56de\u9ed8\u8ba4\u503c<\/strong><\/h3>\n\n\n\n
        SELECT name, IFNULL(age, 0) AS age FROM users;<\/code><\/pre>\n\n\n\n
        \u5982\u679c age<\/code> \u4e3a NULL<\/code>\uff0c\u5219\u8fd4\u56de 0<\/code>\uff0c\u5426\u5219\u8fd4\u56de\u539f\u59cb\u503c\u3002<\/p>\n\n\n\n
        (2) COALESCE()<\/code>\uff1a\u8fd4\u56de\u7b2c\u4e00\u4e2a\u975e NULL<\/code> \u503c<\/strong><\/h3>\n\n\n\n
        SELECT name, COALESCE(age, 18, 20) AS age FROM users;<\/code><\/pre>\n\n\n\n
          \n
        • COALESCE(age, 18, 20)<\/code>\uff1a\u5982\u679c age<\/code> \u662f NULL<\/code>\uff0c\u8fd4\u56de 18<\/code>\uff0c\u5426\u5219\u8fd4\u56de age<\/code>\u3002<\/li>\n<\/ul>\n\n\n\n
          (3) NULLIF()<\/code>\uff1a\u5982\u679c\u4e24\u4e2a\u503c\u76f8\u7b49\uff0c\u5219\u8fd4\u56de NULL<\/code><\/strong><\/h3>\n\n\n\n
          SELECT NULLIF(age, 30) FROM users;<\/code><\/pre>\n\n\n\n
            \n
          • \u5982\u679c age<\/code> \u662f 30<\/code>\uff0c\u5219\u8fd4\u56de NULL<\/code>\uff0c\u5426\u5219\u8fd4\u56de age<\/code>\u3002<\/li>\n<\/ul>\n\n\n\n
            \n\n\n\n
            5. NULL<\/code> \u4e0e ORDER BY<\/code><\/strong><\/h2>\n\n\n\n
              \n
            • NULL<\/code> \u9ed8\u8ba4\u6392\u5e8f\u5728\u6700\u524d\uff1a<\/li>\n<\/ul>\n\n\n\n
                SELECT * FROM users ORDER BY age;<\/code><\/pre>\n\n\n\n
                \n
              • \u8ba9 NULL<\/code> \u6392\u5728\u6700\u540e\uff1a<\/li>\n<\/ul>\n\n\n\n
                  SELECT * FROM users ORDER BY age ASC NULLS LAST;<\/code><\/pre>\n\n\n\n
                  \n
                • \u8ba9 NULL<\/code> \u6392\u5728\u6700\u524d\uff1a<\/li>\n<\/ul>\n\n\n\n
                    SELECT * FROM users ORDER BY age DESC NULLS FIRST;<\/code><\/pre>\n\n\n\n
                  \n\n\n\n
                  6. NULL<\/code> \u5728 GROUP BY<\/code> \u548c HAVING<\/code> \u91cc\u7684\u8868\u73b0<\/strong><\/h2>\n\n\n\n
                    \n
                  • GROUP BY<\/code> \u89c6 NULL<\/code> \u4e3a\u72ec\u7acb\u7ec4\uff1a<\/li>\n<\/ul>\n\n\n\n
                      SELECT age, COUNT(*) FROM users GROUP BY age;<\/code><\/pre>\n\n\n\n
                      \n
                    • NULL<\/code> \u4f5c\u4e3a\u4e00\u7ec4\uff0c\u5355\u72ec\u8ba1\u6570\u3002<\/li>\n\n\n\n
                    • HAVING<\/code> \u9700\u8981 IS NULL<\/code> \u5224\u65ad\uff1a<\/li>\n<\/ul>\n\n\n\n
                        SELECT age, COUNT(*) FROM users GROUP BY age HAVING age IS NULL;<\/code><\/pre>\n\n\n\n
                      \n\n\n\n
                      7. NULL<\/code> \u5728 JOIN<\/code> \u91cc\u7684\u95ee\u9898<\/strong><\/h2>\n\n\n\n
                      \u5982\u679c NULL<\/code> \u53c2\u4e0e JOIN<\/code>\uff0c\u53ef\u80fd\u5bfc\u81f4\u6570\u636e\u4e22\u5931\uff1a<\/p>\n\n\n\n
                      SELECT * FROM users\nLEFT JOIN orders ON users.id = orders.user_id;<\/code><\/pre>\n\n\n\n
                        \n
                      • \u5982\u679c orders.user_id<\/code> \u662f NULL<\/code>\uff0c\u5219\u4e0d\u4f1a\u5339\u914d\u3002<\/li>\n\n\n\n
                      • \u89e3\u51b3\u65b9\u6848\uff1a\u7528 IFNULL()<\/code> \u63d0\u4f9b\u9ed8\u8ba4\u503c\u3002<\/li>\n<\/ul>\n\n\n\n
                        \n\n\n\n
                        \u603b\u7ed3<\/strong><\/h2>\n\n\n\n
                        \u64cd\u4f5c<\/th>\u8bf4\u660e<\/th><\/tr><\/thead>
                        IS NULL<\/code><\/td>\u5224\u65ad\u503c\u662f\u5426\u4e3a NULL<\/code><\/td><\/tr>
                        IS NOT NULL<\/code><\/td>\u5224\u65ad\u503c\u662f\u5426 \u4e0d\u4e3a<\/strong> NULL<\/code><\/td><\/tr>
                        IFNULL(x, y)<\/code><\/td>\u5982\u679c x<\/code> \u662f NULL<\/code>\uff0c\u8fd4\u56de y<\/code><\/td><\/tr>
                        COALESCE(x, y, z, ...)<\/code><\/td>\u8fd4\u56de\u7b2c\u4e00\u4e2a\u975e NULL<\/code> \u503c<\/strong><\/td><\/tr>
                        NULLIF(x, y)<\/code><\/td>\u5982\u679c x = y<\/code>\uff0c\u8fd4\u56de NULL<\/code>\uff0c\u5426\u5219\u8fd4\u56de x<\/code><\/td><\/tr>
                        ORDER BY ... NULLS LAST\/FIRST<\/code><\/td>\u63a7\u5236 NULL<\/code> \u5728\u6392\u5e8f\u4e2d\u7684\u4f4d\u7f6e<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
                        \n\n\n\n
                        \u66f4\u591a\u8be6\u7ec6\u5185\u5bb9\u8bf7\u5173\u6ce8\u5176\u4ed6\u76f8\u5173\u6587\u7ae0\u3002<\/p>\n","protected":false},"excerpt":{"rendered":"
                        \u5728 SQLite \u4e2d\uff0cNULL \u662f\u4e00\u79cd\u7279\u6b8a\u503c\uff0c\u8868\u793a “\u672a\u77e5” \u6216 “\u4e0d\u5b58\u5728 […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[76],"tags":[],"class_list":["post-2356","post","type-post","status-publish","format-standard","hentry","category-sqlite"],"_links":{"self":[{"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/posts\/2356","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/comments?post=2356"}],"version-history":[{"count":1,"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/posts\/2356\/revisions"}],"predecessor-version":[{"id":2357,"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/posts\/2356\/revisions\/2357"}],"wp:attachment":[{"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/media?parent=2356"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/categories?post=2356"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/tags?post=2356"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}