{"id":15682,"date":"2024-07-09T14:28:17","date_gmt":"2024-07-09T06:28:17","guid":{"rendered":"https:\/\/92it.top\/?p=15682"},"modified":"2024-07-09T14:28:17","modified_gmt":"2024-07-09T06:28:17","slug":"count-vs-count1-%e5%a6%82%e4%bd%95%e9%80%89%ef%bc%9f","status":"publish","type":"post","link":"https:\/\/92it.top\/?p=15682","title":{"rendered":"count(*) vs count(1) \u5982\u4f55\u9009\uff1f"},"content":{"rendered":"\n

\u8f6c\u8f7d\uff1acount(*) vs count(1) \u5982\u4f55\u9009\uff1f<\/a><\/p>\n\n\n\n

\u5728\u5b9e\u9645\u5f00\u53d1\u5de5\u4f5c\u4e2d\uff0c\u96be\u514d\u4f1a\u4f7f\u7528\u5230 MySQL \u7684 count() \u805a\u5408\u51fd\u6570\u8fdb\u884c\u7edf\u8ba1\u64cd\u4f5c\uff0c\u5bf9\u4e8ecount(1)\u3001count(*)\u3001count(\u5e38\u91cf)\u3001count(\u4e3b\u952e)\u3001count(\u975e\u4e3b\u952e)\u3001count(distinct(\u5b57\u6bb5)) \u7b49\u591a\u4e2a\u51fd\u6570\uff0c\u5f88\u591a\u5f00\u53d1\u4eba\u5458\u56e0\u4e3a\u7f3a\u4e4f\u539f\u7406\u6027\u7684\u4e86\u89e3\uff0c\u5f80\u5f80\u4f1a\u6bd4\u8f83\u56f0\u60d1\u9009\u62e9\u54ea\u4e00\u79cd\uff0c\u7279\u522b\u662f\u5728\u52a0 where \u6761\u4ef6\u65f6\uff0c\u66f4\u52a0\u62c5\u5fc3\u4f1a\u4e0d\u4f1a\u9020\u6210\u6027\u80fd\u95ee\u9898\uff0c\u4eca\u5929\u6211\u4eec\u5c31\u6765\u804a\u804a\u8fd9\u4e9b count() \u51fd\u6570\u80cc\u540e\u7684\u5b9e\u73b0\u539f\u7406\u4ee5\u53ca\u6267\u884c\u6548\u7387\u3002<\/p>\n\n\n\n

\u7533\u660e\uff1a\u672c\u6587\u57fa\u4e8e MySQL 8.0.30\uff0c\u6570\u636e\u5e93\u5f15\u64ce\u4e3a InnoDB \u5f15\u64ce \u548c MyISAM \u5f15\u64ce\uff1bMySQL 8.0.30 \u9ed8\u8ba4\u4f7f\u7528 InnoDB \u6570\u636e\u5e93\u5f15\u64ce \u3002<\/p>\n\n\n\n

\u4e00\u3001\u524d\u8a00\u00a0<\/strong><\/p>\n\n\n\n

\n\n\n\n

count(expr) \u662f MySQL \u7684\u4e00\u4e2a\u805a\u5408\u51fd\u6570\uff0c\u51fd\u6570\u63a5\u6536

\u4e00\u4e2a expr \u8868\u8fbe\u5f0f\uff0cexpr \u8868\u8fbe\u5f0f\u53ef\u4ee5\u662f NULL\u3001

\u53ef\u4ee5\u662f\u5217\u540d\uff0c\u53ef\u4ee5\u662f\u5e38\u91cf\uff0c\u53ef\u4ee5\u662f\u5176\u4ed6\u4efb\u610f\u8868\u8fbe\u5f0f\u90fd\u53ef\u4ee5\uff0c

\u5b83\u6709 2\u4e2a\u91cd\u8981\u7684\u4f5c\u7528\uff1a\u7edf\u8ba1\u67d0\u4e2a\u5217\u503c\u7684\u6570\u91cf\u4ee5\u53ca\u7edf\u8ba1

\u8868\u884c\u6570\u3002\u5728\u7edf\u8ba1\u5217\u6570\u91cf\u65f6\u53ea\u7edf\u8ba1\u5217\u503c\u4e0d\u4e3a NULL \u7684

\u603b\u6570\uff0c\u7edf\u8ba1\u8868\u603b\u884c\u6570\u65f6\u4e0d\u505a\u9650\u5236\u3002

\u4e3a\u4e86\u66f4\u597d\u5730\u5c55\u5f00\u672c\u6587\u7684\u8bb2\u89e3\uff0c\u9700\u8981\u5148\u521b\u5efa user \u548c person \u4e24\u5f20\u8868\uff0cuser \u8868\u4f7f\u7528 InnoDB \u5f15\u64ce\uff0cperson \u8868\u4f7f\u7528 MyISAM \u5f15\u64ce\uff0c\u8868\u521b\u5efa\u5b8c\u6210\u540e\uff0c\u67e5\u770b\u5b83\u4eec\u5728\u78c1\u76d8\u4e0a\u7684\u6587\u4ef6\u4fe1\u606f\u5bf9\u6bd4\uff0c\u5177\u4f53\u5bf9\u6bd4\u5982\u4e0b\u622a\u56fe\uff1a<\/p>\n\n\n

\n
\"\"<\/figure><\/div>\n\n\n

\u4ece\u4e0a\u8ff0\u622a\u56fe\u53ef\u4ee5\u770b\u51fa\uff1a\u4f7f\u7528 MyISAM \u5f15\u64ce\u7684 person \u8868\uff0c\u8868\u5b9a\u4e49\uff0c\u6570\u636e\uff0c\u7d22\u5f15\u662f\u5206\u4e09\u4e2a\u6587\u4ef6\u5b58\u50a8\uff0c\u6587\u4ef6\u8bf4\u660e\u5982\u4e0b\uff1a<\/p>\n\n\n

\n
\"\"<\/figure><\/div>\n\n\n

\u4f7f\u7528 InnoDB \u5f15\u64ce\u7684 user \u8868\uff0c\u8868\u5b9a\u4e49\uff0c\u6570\u636e\uff0c\u7d22\u5f15\u90fd\u5b58\u653e\u5728\u4e00\u4e2a\u6587\u4ef6\u4e2d\uff0c\u6587\u4ef6\u8bf4\u660e\u5982\u4e0b\uff1a<\/p>\n\n\n

\n
\"\"<\/figure><\/div>\n\n\n

\u4ece\u4e0a\u9762\u8868\u7684\u521b\u5efa\u53ef\u4ee5\u53d1\u73b0 InnoDB \u5f15\u64ce \u548c MyISAM \u5f15\u64ce\u5728\u8868\u6587\u4ef6\u4e0a\u7684\u5dee\u5f02\u6027\uff0c\u63a5\u4e0b\u6765\u4f1a\u56f4\u7ed5\u8fd9\u4e24\u79cd\u6570\u636e\u5e93\u5f15\u64ce\u6765\u5206\u6790\u5404\u79cd count(expr) \u64cd\u4f5c\u3002<\/p>\n\n\n\n

\u4e8c\u3001count(*)\u00a0<\/strong><\/p>\n\n\n\n

\n\n\n\n

\u5bf9\u4e8e MyISAM \u5f15\u64ce\uff0c\u4f1a\u628a\u8868\u7684\u603b\u884c\u6570\u5b58\u5728\u4e86\u78c1\u76d8\u4e0a\uff08\u5b58\u653e\u5728 information_schema \u5e93\u4e2d\u7684 PARTITIONS \u8868\u4e2d\uff09\uff0c\u5728\u4e0d\u52a0 where \u6761\u4ef6\u65f6\uff0c\u6267\u884c count(*) \u65f6\u4f1a\u76f4\u63a5\u8fd4\u56de\u8fd9\u4e2a\u603b\u6570\uff0c\u56e0\u6b64\u6548\u7387\u5f88\u9ad8\uff0c\u4f46\u662f\u5728\u52a0 where \u9650\u5b9a\u8bed\u53e5\u7684\u65f6\u5019 MySQL \u9700\u8981\u5bf9\u5168\u8868\u8fdb\u884c\u68c0\u7d22\u4ece\u800c\u5f97\u51fa count \u7684\u603b\u6570\u3002<\/p>\n\n\n\n

\u800c InnoDB \u5f15\u64ce\u5e76\u6ca1\u6709\u50cf MyISAM \u90a3\u6837\u628a\u8868\u7684\u603b\u884c\u6570\u5b58\u50a8\u5728\u78c1\u76d8\uff0c\u800c\u662f\u5728\u6267\u884c count(* )\u65f6\uff0c\u5728\u4e0d\u52a0 where \u9650\u5b9a\u8bed\u53e5\u65f6\uff0cMySQL Server \u5c42\u9700\u8981\u628a\u6570\u636e\u4ece\u5f15\u64ce\u91cc\u9762\u8bfb\u51fa\u6765\uff0c\u7136\u540e\u9010\u884c\u7d2f\u52a0\u5f97\u51fa\u603b\u6570\uff1b\u5982\u679c\u52a0\u4e86 where \u9650\u5b9a\u8bed\u53e5\uff0c\u9700\u8981\u6839\u636e where \u6761\u4ef6\u4ece\u5f15\u64ce\u91cc\u9762\u7b5b\u9009\u51fa\u6570\u636e\uff0c\u7136\u540e\u7d2f\u52a0\u5f97\u51fa\u603b\u6570<\/p>\n\n\n\n

\u4e0b\u56fe\u5c55\u793a\u4e86 MyISAM \u5f15\u64ce\u78c1\u76d8\u5b58\u50a8\u6570\u636e\u603b\u884c\u6570<\/p>\n\n\n

\n
\"\"<\/figure><\/div>\n\n\n

\u4e3a\u4e86\u66f4\u597d\u5730\u7406\u89e3\u5728\u4e24\u79cd\u5f15\u64ce\u4e2d\u6267\u884c count(*) \u7684\u5dee\u5f02\uff0c\u6211\u4eec\u6bd4\u8f83\u4e86 count(*) \u5728 user \u8868\u548c person \u8868\u4e2d\u7684\u6267\u884c\u8ba1\u5212\uff0c\u7ed3\u679c\u5982\u4e0b\u56fe\uff1a<\/p>\n\n\n

\n
\"\"<\/figure><\/div>\n\n\n

\u4ece\u4e0a\u8ff0 explain \u6267\u884c\u8ba1\u5212\u622a\u56fe\u53ef\u4ee5\u770b\u51fa\uff1a<\/p>\n\n\n\n

 \u4f7f\u7528 InnoDB \u5f15\u64ce\u7684 user \u8868\u7684\u6267\u884c\u8ba1\u5212\u663e\u793a\uff1arows = 2\uff08\u5168\u8868\u53ea\u6709 2 \u884c\u6570\u636e\uff09\uff0c\u8bf4\u660e\u9700\u8981\u626b\u63cf\u5168\u8868\uff0cExtra \u91cc\u9762\u7684\u5185\u5bb9\u662f \"Using index\"\uff0c\u8bf4\u660e\u8be5 count( )\u64cd\u4f5c\u4f7f\u7528\u4e86\u7d22\u5f15\uff0ckey\uff0c\u8bf4\u660e\u662f\u4f7f\u7528\u4e86 age\u5b57\u6bb5\u7d22\u5f15\uff0c\u6b64\u5904\u4f7f\u7528 age\u5b57\u6bb5\u7d22\u5f15\u800c\u672a\u4f7f\u7528\u4e3b\u952e\u7d22\u5f15\uff0c\u7406\u7531\u4f1a\u5728\u4e0b\u6587\u5206\u6790\u3002


\u4f7f\u7528 MyISAM \u5f15\u64ce\u7684 person \u8868\u7684\u6267\u884c\u8ba1\u5212\u663e\u793a\uff1a rows = NULL\uff0c\u8bf4\u660e\u4e0d\u9700\u8981\u626b\u63cf\u8868\uff0cExtra \u91cc\u9762\u7684\u5185\u5bb9\u662f \"Select tables optimized away\"\uff0c\u5b83\u7684\u610f\u601d\u662f\uff1aMyISAM \u8868\u4ee5\u5355\u72ec\u7684\u884c\u6570\u5b58\u50a8\u603b\u6570\uff0c\u6267\u884c count \u67e5\u8be2\u65f6\uff0cMySQL \u4e0d\u9700\u8981\u67e5\u770b\u4efb\u4f55\u8868\u884c\u6570\u636e\uff0c\u800c\u662f\u5c06\u9884\u5148\u8ba1\u7b97\u7684\u884c\u6570\u7acb\u5373\u8fd4\u56de\u3002
<\/pre>\n\n\n\n
\u90a3\u4e48\uff0c\u4e3a\u4ec0\u4e48 InnoDB \u5f15\u64ce\u4e0d\u80fd\u50cf MyISAM \u5f15\u64ce\u4e00\u6837\uff0c\u628a\u8868\u7684\u6570\u636e\u603b\u6570\u5b58\u8d77\u6765\uff0c\u800c\u662f\u9700\u8981\u626b\u63cf\u5168\u8868\u5462\uff1f<\/p>\n\n\n\n
\u8fd9\u662f\u56e0\u4e3a InnoDB \u5f15\u64ce\u9700\u8981\u652f\u6301\u4e8b\u52a1\uff0c\u9ed8\u8ba4\u7684\u9694\u79bb\u7ea7\u522b\u662f Repeatable Read\uff08\u53ef\u91cd\u590d\u8bfb\uff0c\u6307\u7684\u662f\u4e00\u4e2a\u4e8b\u52a1\u6267\u884c\u8fc7\u7a0b\u65e0\u6cd5\u770b\u5230\u5176\u5b83\u4e8b\u52a1\u672a\u63d0\u4ea4\u7684\u6570\u636e\uff09\uff0c\u800c\u53ef\u91cd\u590d\u8bfb\u53c8\u662f\u901a\u8fc7\u591a\u7248\u672c\u5e76\u53d1\u63a7\u5236\uff08MVCC\uff09\u6765\u5b9e\u73b0\u7684\uff0cMVCC \u66f4\u76f4\u767d\u7684\u8868\u8ff0\u662f\uff1a\u4e00\u884c\u8bb0\u5f55\u5728\u4e0d\u540c\u7684\u4e8b\u52a1\u4e2d\u8868\u73b0\u7684\u7ed3\u679c\u503c\u662f\u4e0d\u4e00\u6837\u7684\uff0c\u5448\u73b0\u51fa\u4e00\u884c\u8bb0\u5f55\u591a\u79cd\u7248\u672c\u6570\u636e\u3002\u5173\u4e8e MVCC \u53ef\u4ee5\u53c2\u7167\u4e0b\u9762\u7684\u56fe\u6765\u7406\u89e3\uff1a<\/p>\n\n\n
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\"\"<\/figure><\/div>\n\n\n
\u4e3a\u4e86\u66f4\u597d\u5730\u8bf4\u660e InnoDB \u5f15\u64ce\u4e3a\u4ec0\u4e48\u4e0d\u628a\u6570\u636e\u603b\u884c\u6570\u5b58\u78c1\u76d8\uff0c\u8fd9\u91cc\u901a\u8fc7\u4e00\u4e2a\u6848\u4f8b\u8fdb\u884c\u89e3\u6790\uff1a\u5047\u8bbe\u6709\u4e24\u4e2a\u4e8b\u52a1 sessionA \u548c sessionB\uff0csessionA \u5148\u542f\u52a8\u4e00\u4e2a\u4e8b\u52a1\uff0c\u7136\u540e count(*) user\u8868\u7edf\u8ba1\u603b\u884c\u6570\uff1bsessionB \u542f\u52a8\u4e8b\u52a1\uff0c\u5148\u6267\u884c\u4e00\u6b21 count(*) user \u8868\uff0c\u7136\u540e\u65b0\u589e\u4e00\u884c\u8bb0\u5f55\uff0c\u518d count(*) user\u8868\u7edf\u8ba1\u603b\u884c\u6570\uff1b\u6574\u4e2a\u6267\u884c\u987a\u5e8f\u6d41\u4ee5\u53ca\u622a\u56fe\u5982\u4e0b\uff1a<\/p>\n\n\n
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\u901a\u8fc7\u8fd0\u884c\u7ed3\u679c\u622a\u56fe\uff0c\u6211\u4eec\u53ef\u4ee5\u770b\u51fa\uff0csessionB \u65b0\u589e\u4e00\u6761\u8bb0\u5f55\u540e\uff0c\u5728\u672a commit \u63d0\u4ea4\u4e8b\u52a1\u524d\uff0csessionA \u7684 count(*) \u64cd\u4f5c\u5e76\u6ca1\u6709\u628a\u8fd9\u6761\u6570\u636e\u7edf\u8ba1\u8fdb\u53bb\uff0c\u7b26\u5408\u53ef\u91cd\u590d\u8bfb\u9694\u79bb\u7ea7\u522b\u7684\u8981\u6c42\uff0c\u5047\u5982 InnoDB \u4e5f\u50cf MyISAM \u4e00\u6837\u628a\u884c\u7684\u603b\u6570\u5b58\u5728\u78c1\u76d8\u4e0a\uff0c\u90a3\u4e48 sessionA \u548c sessionB \u7684 count(*) \u7ed3\u679c\u503c\u76f8\u540c\uff0c\u4e5f\u5c31\u662f\u8bf4 sessionA \u548c sessionB \u6700\u540e\u4e00\u6b21 count(*) \u7684\u7ed3\u679c\u503c\u90fd\u662f 3\uff0c\u8fd9\u663e\u7136\u5c31\u8fdd\u80cc\u4e86\u53ef\u91cd\u590d\u8bfb\u9694\u79bb\u7ea7\u522b\u7684\u8981\u6c42\u3002<\/p>\n\n\n\n
\u6240\u4ee5\uff0c\u901a\u8fc7\u8be5\u6848\u4f8b\u7684\u5206\u6790\u4e5f\u521a\u597d\u9a8c\u8bc1\u4e86\u4e0a\u8ff0 explain count(*) \u7684\u6267\u884c\u8ba1\u5212\uff0cInnoDB\u5f15\u64ce\u9700\u8981\u5168\u8868\u626b\u63cf\u3002<\/p>\n\n\n\n
\u6709\u4eba\u8bf4\uff0c\u5728\u4f7f\u7528 InnoDB \u5f15\u64ce\u7684 user\u8868\u4e0a\u6267\u884c “show table status” \u6307\u4ee4\uff0c\u7ed3\u679c\u4e2d\u7684 ROWS \u5b57\u6bb5\u5c31\u662f\u8868\u7684\u603b\u884c\u6570\uff0c\u5feb\u6377\u65b9\u4fbf\uff0c\u6548\u7387\u9ad8\u3002\u90a3\u4e48\uff0c\u53ef\u884c\u5417\uff1f<\/strong><\/p>\n\n\n\n
\u65b9\u6cd5\u662f\u5426\u6709\u6548\uff0c\u8fd8\u662f\u5f97\u7528\u4e8b\u5b9e\u8bf4\u8bdd\uff0c\u6211\u4eec\u53ef\u4ee5\u6267\u884c “show table status” \u6307\u4ee4\uff0c\u6267\u884c\u987a\u5e8f\u6d41\u548c\u7ed3\u679c\u622a\u56fe\u5982\u4e0b\uff1a<\/p>\n\n\n
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\u901a\u8fc7\u8fd0\u884c\u7ed3\u679c\u622a\u56fe\u53ef\u4ee5\u770b\u51fa\uff1asessionA \u7684 count(*) \u7ed3\u679c\u548c “show table status” \u6307\u4ee4\u7ed3\u679c\u4e2d\u7684 ROWS \u5b57\u6bb5\u503c\u76f8\u7b49\uff0c\u4f46\u662f sessionB \u4e2d\u4e24\u4e2a\u503c\u5c31\u4e0d\u4e00\u6837\uff0c\u56e0\u6b64\u8bf4\uff0c\u901a\u8fc7 “show table status” \u6765\u7edf\u8ba1 InnoDB \u8868\u7684\u603b\u884c\u6570\uff0c\u7ed3\u679c\u503c\u662f\u4e0d\u51c6\u786e\u7684\u3002<\/p>\n\n\n\n
\u6309\u7167 MySQL \u5b98\u65b9\u7684\u8bf4\u6cd5\uff1a”show table status” \u547d\u4ee4\u7ed3\u679c\u503c\u4e2d ROWS \u5b57\u6bb5\u663e\u793a\u7684\u884c\u6570\u8bef\u5dee\u7387\u5728 40% ~ 50%\u3002<\/p>\n\n\n\n
show table status \u67e5\u8be2\u7684\u662f MySQL \u670d\u52a1\u81ea\u5e26\u7684 information_schema \u5e93\u4e2d\u7684 TABLES \u8868\uff0c\u8be6\u60c5\u53c2\u8003TABLES \u5b98\u65b9\u6587\u6863 \uff1a<\/p>\n\n\n\n
https:\/\/docs.oracle.com\/cd\/E17952_01\/mysql-8.0-en\/information-schema-partitions-table.html<\/a><\/p>\n\n\n\n
\u9700\u8981\u8bf4\u660e\u7684\u662f\uff1a\u5728\u4e0a\u6587 explain \u6267\u884c\u8ba1\u5212\u4e2d\u53ef\u4ee5\u53d1\u73b0\uff0c\u5c3d\u7ba1 InnoDB \u5f15\u64ce\u7684 count(*) user \u8868\u64cd\u4f5c\u9700\u8981\u626b\u63cf\u5168\u8868\uff0c\u4f46\u662f MySQL \u8fd8\u662f\u6709\u505a\u8fc7\u4f18\u5316\u5904\u7406\uff0c\u4f7f\u7528\u4e86\u7d22\u5f15\uff0c\u5177\u4f53\u4f18\u5316\u5982\u4e0b\uff1a<\/p>\n\n\n\n
\u56e0\u4e3a InnoDB \u5f15\u64ce\u91c7\u7528\u7684\u662f\u805a\u7c07\u7d22\u5f15\u673a\u5236\uff0c\u4e3b\u952e\u7d22\u5f15\u7684\u53f6\u5b50\u8282\u70b9\u5b58\u653e\u4e86\u6570\u636e\uff0c\u800c\u666e\u901a\u7d22\u5f15\u7684\u53f6\u5b50\u8282\u70b9\u5b58\u653e\u7684\u662f\u4e3b\u952e\u503c\u3002\u56e0\u6b64\uff0c\u4e0d\u7ba1\u904d\u5386\u54ea\u4e00\u68f5\u7d22\u5f15\u6811\uff0ccount( * )\u7684\u7ed3\u679c\u90fd\u662f\u4e00\u81f4\u7684\uff0c\u6240\u4ee5\uff0cMySQL \u4f18\u5316\u5668\u4f1a\u627e\u5230\u6700\u5c0f\u7684\u90a3\u68f5\u7d22\u5f15\u6811\u8fdb\u884c\u904d\u5386\uff0c\u8fd9\u6837\u5c3d\u7ba1\u626b\u63cf\u7684\u884c\u6570\u6ca1\u6709\u51cf\u5c11\uff0c\u4f46\u662f\u9488\u5bf9\u6bcf\u884c\u8bb0\u5f55\u83b7\u53d6\u7684\u6570\u636e\u91cf\u51cf\u5c11\u4e86\uff0c\u56e0\u6b64\u6027\u80fd\u5c31\u63d0\u5347\u4e86\u3002\u56e0\u6b64\u4e0a\u6587explain \u6267\u884c\u8ba1\u5212\u4f7f\u7528 age \u5b57\u6bb5\u7d22\u5f15\u800c\u975e\u4e3b\u952e\u7d22\u5f15\u3002<\/p>\n\n\n\n
\u6709\u4e86\u4e0a\u8ff0\u5bf9 count(*) \u7684\u8bb2\u89e3\uff0c\u6211\u4eec\u5206\u6790\u548c\u7406\u89e3\u5176\u4ed6\u51e0\u79cd count(exprexpr) \u64cd\u4f5c\u5c31\u4f1a\u8f7b\u677e\u5f88\u591a\u3002<\/p>\n\n\n\n
\u4e09\u3001count(1)\u00a0<\/strong><\/p>\n\n\n\n
\n\n\n\n
\u5bf9\u4e8e MyISAM \u5f15\u64ce\uff0ccount(1) \u548c count(*) \u7684 \u903b\u8f91\u662f\u4e00\u6837\u7684\u3002<\/p>\n\n\n\n
\u5bf9\u4e8e InnoDB \u5f15\u64ce\uff0c\u6309\u7167\u5b98\u65b9\u6587\u6863\uff0ccount(1) \u548c count(*) \u7684\u5904\u7406\u65b9\u5f0f\u4e00\u6837\uff0c\u65e0\u6027\u80fd\u5dee\u522b\uff0c\u5b98\u65b9\u6587\u6863\u622a\u56fe\u5982\u4e0b\uff1a<\/p>\n\n\n
\n
\"\"<\/figure><\/div>\n\n\n
\u56db\u3001count(\u5e38\u91cf)\u00a0<\/strong><\/p>\n\n\n\n
\n\n\n\n
count(\u5e38\u91cf)\u7684\u6267\u884c\u903b\u8f91\u548c count(1) \u7684\u903b\u8f91\u662f\u4e00\u6837\u7684\uff0c\u6bd4\u5982\uff1acount(5)\u3001count(‘abc’)\u3001count(‘.’)\uff0c\u5177\u4f53\u7ed3\u679c\u53c2\u7167\u4e0b\u56fe\uff1a<\/p>\n\n\n
\n
\"\"<\/figure><\/div>\n\n\n
\u4e94\u3001count(\u4e3b\u952e)\u00a0<\/strong><\/p>\n\n\n\n
\n\n\n\n
\u5bf9\u4e8e MyISAM \u5f15\u64ce\uff0c\u4e0d\u52a0 where \u6761\u4ef6\u65f6\uff0c\u76f4\u63a5\u8fd4\u56de\u7684\u8868\u4e2d\u4fdd\u5b58\u7684\u6570\u636e\u603b\u884c\u6570\u503c\uff1b\u52a0 where \u6761\u4ef6\u65f6\uff0c\u9700\u8981\u8d70\u4e3b\u952e\u7d22\u5f15\u7b5b\u9009\u51fa\u503c\u540e\u518d\u7edf\u8ba1\uff1b<\/p>\n\n\n\n
\u5bf9\u4e8e InnoDB \u5f15\u64ce\uff0c\u4e0d\u52a0 where \u6761\u4ef6\u65f6\uff0ccount(id) \u548c count(*) \u7684\u5904\u7406\u65b9\u5f0f\u4e00\u6837\uff0cMySQL\u4f18\u5316\u5668\u4f1a\u9009\u62e9\u6700\u5c0f\u6811\u7d22\u5f15age \u7d22\u5f15\u8fdb\u884c\u904d\u5386\u7edf\u8ba1\uff1b\u52a0 where \u6761\u4ef6\u65f6\uff0c\u9700\u8981\u8d70\u4e3b\u952e\u7d22\u5f15\u7b5b\u9009\u51fa\u503c\u540e\u518d\u7edf\u8ba1\uff1b<\/p>\n\n\n\n
\u5b9e\u4f8b\u622a\u56fe\u5982\u4e0b\uff1a<\/p>\n\n\n
\n
\"\"<\/figure><\/div>\n\n\n
\u516d\u3001count(\u975e\u4e3b\u952e)\u00a0<\/strong><\/p>\n\n\n\n
\n\n\n\n
count(\u975e\u4e3b\u952e\u5b57\u6bb5) \u64cd\u4f5c\u6709\u4e9b\u7279\u6b8a\uff0c\u6211\u4eec\u5148\u770b\u4e00\u5f20\u5b9e\u4f8b\u622a\u56fe\uff1a<\/p>\n\n\n
\n
\"\"<\/figure><\/div>\n\n\n
\u5982\u4e0a\u8ff0\u622a\u56fe\uff1a\u4f7f\u7528 InnoDB \u5f15\u64ce\u7684 user \u8868\u6700\u5f00\u59cb\u6709 3 \u6761\u6570\u636e\uff0c\u7136\u540e\u65b0\u589e\u4e00\u6761 age= NULL \u7684\u8bb0\u5f55\uff0c\u5404\u79cd count(expr) \u7684\u7ed3\u679c\u4e3a\uff1a<\/p>\n\n\n
\n
\"\"<\/figure><\/div>\n\n\n
\u4f7f\u7528 MyISAM \u5f15\u64ce\u7684 person \u8868\u6700\u5f00\u59cb\u6709 3 \u6761\u6570\u636e\uff0c\u7136\u540e\u65b0\u589e\u4e00\u6761 name=NULL \u7684\u8bb0\u5f55\uff0c\u5404\u79cd count(expr) \u7684\u7ed3\u679c\u4e3a\uff1a<\/p>\n\n\n
\n
\"\"<\/figure><\/div>\n\n\n
\u4ece\u7ed3\u679c\u53ef\u4ee5\u53d1\u73b0\uff0c\u4e0d\u7ba1\u662f\u4f7f\u7528\u4e86InnoDB \u5f15\u64ce\u7684 user \u8868\uff0c\u8fd8\u662f\u4f7f\u7528\u4e86 MyISAM\u5f15\u64ce\u7684 person \u8868\uff0c\u901a\u8fc7 count(\u975e\u4e3b\u952e)\u65b9\u5f0f\u8fdb\u884c\u7edf\u8ba1\u65f6\uff0c\u8868\u7684\u603b\u884c\u6570\u5c31\u4f1a\u6bd4\u5176\u4ed6\u51e0\u79cd count(expr)\u65b9\u5f0f\u5c11 1 \u6761\uff0c\u4e3a\u4ec0\u4e48\u5462\uff1f<\/p>\n\n\n\n
\u8fd9\u662f\u56e0\u4e3a\uff1acount(\u975e\u4e3b\u952e) \u53ea\u7edf\u8ba1\u975e\u4e3b\u952e\u5b57\u6bb5\u503c\u4e0d\u4e3aNULL\u7684\u603b\u6570\u3002<\/strong><\/p>\n\n\n\n
\u53e6\u5916\uff0ccount(\u975e\u4e3b\u952e)\u4e2d\u975e\u4e3b\u952e\u5b57\u6bb5\u662f\u5426\u589e\u52a0\u4e86\u7d22\u5f15\u5bf9\u7edf\u8ba1\u6027\u80fd\u5f71\u54cd\u4e5f\u662f\u5f88\u5927\uff0c\u5177\u4f53\u5982\u4e0b\uff1a<\/p>\n\n\n\n
InnoDB \u5f15\u64ce\u7684user\u8868\uff1a<\/p>\n\n\n
\n
\"\"<\/figure><\/div>\n\n\n
MyISAM\u5f15\u64ce\u7684 person\u8868\uff1a<\/p>\n\n\n
\n
\"\"<\/figure><\/div>\n\n\n
\u901a\u8fc7\u4e0a\u8ff0\u622a\u56fe\u53ef\u4ee5\u603b\u7ed3\u51fa\uff1acount(\u975e\u4e3b\u952e)\u4e0d\u7ba1\u5728 InnoDB \u5f15\u64ce \u8fd8\u662f\u5728 MyISAM\u5f15\u64ce\u4e2d\u6267\u884c\uff0c\u975e\u4e3b\u952e\u8fd9\u4e2a\u5b57\u6bb5\u662f\u5426\u6dfb\u52a0\u4e86\u7d22\u5f15\u76f4\u63a5\u5f71\u54cd\u4e86count() \u7edf\u8ba1\u626b\u63cf\u8868\u7684\u884c\u6570\uff0c\u4ece\u800c\u5f71\u54cd\u7edf\u8ba1\u7684\u6027\u80fd\u3002<\/p>\n\n\n\n
\u4e03\u3001count(distinct(\u5b57\u6bb5))\u00a0<\/strong><\/p>\n\n\n\n
count(distinct(\u5b57\u6bb5)) \u5176\u5b9e\u662f count(\u5b57\u6bb5) + distinct \u7684\u7ed3\u679c\u96c6\uff0c\u7edf\u8ba1\u5b57\u6bb5\u4e0d\u4e3aNULL\uff0c\u5e76\u4e14\u5728\u5b57\u6bb5\u503c\u91cd\u590d\u7684\u60c5\u51b5\u4e0b\u53ea\u7edf\u8ba1\u4e00\u6b21\uff0c\u4e0b\u9762\u7ed9\u51fa\u4e00\u5f20\u5b9e\u4f8b\u622a\u56fe\uff1a<\/pre>\n\n\n
\n
\"\"<\/figure><\/div>\n\n\n
\u516b\u3001\u6e90\u7801\u7b80\u6790\u00a0<\/strong><\/p>\n\n\n\n
\n\n\n\n
\u4ece\u4e0a\u9762\u7684\u7406\u8bba\u5206\u6790\u540e\uff0c\u6211\u4eec\u518d\u4ece\u6e90\u7801\u89d2\u5ea6\u770b\u4e0b MySQL \u662f\u5982\u4f55\u5b9e\u73b0 count(expr) \u7684\uff0cMySQL\u7684\u6e90\u7801\u662f C++\u5b9e\u73b0\u7684\u3002count() \u51fd\u6570\u7684\u5b9e\u73b0\u6e90\u7801\u662f item_sum.cc \u6587\u4ef6\uff0c\u5728 server \u7aef\u7ef4\u62a4\u4e86\u4e00\u4e2a count \u53d8\u91cf\u6765\u8bb0\u5f55 count(expr) \u7684\u7ed3\u679c\u503c\uff0c\u6e90\u7801\u622a\u56fe\u5982\u4e0b<\/p>\n\n\n
\n
\"\"<\/figure><\/div>\n\n\n
\u63a5\u7740\u770b\u4e0b arg_is_null() \u51fd\u6570\u7684\u6e90\u7801\u5b9e\u73b0\uff0c\u622a\u56fe\u5982\u4e0b\uff1a<\/p>\n\n\n
\n
\"\"<\/figure><\/div>\n\n\n
\u5176\u4e2d item[i] \u8868\u793a\u7684\u5c31\u662f count(expr) \u51fd\u6570\u4e2d expr \u8868\u8fbe\u5f0f\u7684\u5185\u5bb9\uff0c\u53ef\u4ee5\u901a\u8fc7\u8c03\u8bd5\u770b\u4e0b item[i] \u7684\u503c\u662f\u4ec0\u4e48\u3002<\/p>\n\n\n\n
count(*) \u8c03\u8bd5\u7ed3\u679c\u5982\u4e0b\uff1a<\/strong><\/p>\n\n\n
\n
\"\"<\/figure><\/div>\n\n\n
\u4e0a\u8ff0\u622a\u56fe\u53ef\u4ee5\u770b\u51fa\uff0c\u5f53expr=* \u65f6\uff0c\u5728MySQL\u6e90\u7801\u4e2d\u4f1a\u628a*\u7684\u503c\u5f53\u505a longlong \u7c7b\u578b\u76840 \u5904\u7406\uff0c\u56e0\u6b64 count(*) \u5c31\u8f6c\u6362\u6210 count(0) \u4e86\u3002<\/p>\n\n\n\n
count(1) \u8c03\u8bd5\u7ed3\u679c\u5982\u4e0b\uff1a<\/strong><\/p>\n\n\n
\n
\"\"<\/figure><\/div>\n\n\n
\u5728MySQL\u6e90\u7801\u4e2d\u4f1a\u628acount(1)\u4e2d\u7684expr=1\u76f4\u63a5\u5f53\u505a longlong \u7c7b\u578b\u76841 \u5904\u7406\uff0c\u89e3\u91ca\u4e86 count(1)\uff0c\u4e5f\u5c31\u5f88\u597d\u7406\u89e3 count(\u6570\u5b57\u5e38\u91cf)\u8fd9\u4e9b\u7edf\u8ba1\u64cd\u4f5c\u4e86\u3002<\/p>\n\n\n\n
count(\u5b57\u6bb5\u540d) \u8c03\u8bd5\u7ed3\u679c\u5982\u4e0b\uff1a<\/strong><\/p>\n\n\n
\n
\"\"<\/figure><\/div>\n\n\n
\u901a\u8fc7\u622a\u56fe\u53ef\u4ee5\u770b\u5230\uff0c\u5f53 expr=\u5b57\u6bb5\u540d\uff0c\u5bf9\u5e94\u7684\u7c7b\u578b\u662fItem_field\uff0c\u4ee3\u8868\u4e00\u4e2a\u5b57\u6bb5\u3002<\/p>\n\n\n\n
\u6e29\u99a8\u63d0\u793a\uff1a\u56e0\u4e3a\u5386\u53f2\u539f\u56e0\uff0cC++\u4e2d\u7684 longint\u76f8\u5f53\u4e8eJava\u4e2d\u7684int\uff0clonglong\u5176\u5b9e\u76f8\u5f53\u4e8eJava\u4e2d\u7684long\u3002<\/p>\n\n\n\n
\u6700\u540e\u5c06InnoDB count(expr)\u6574\u4e2a\u8fc7\u7a0b\u7b3c\u7edf\u5730\u5f52\u7eb3\u4e3a\u4e0b\u56fe\uff1a<\/p>\n\n\n
\n
\"\"<\/figure><\/div>\n\n\n
\u4e5d\u3001\u603b\u7ed3\u00a0<\/strong><\/p>\n\n\n\n
\n\n\n\n
count(expr)\u51fd\u6570\u7684\u53c2\u6570 expr\u53ef\u4ee5\u662f\u4efb\u610f\u7684\u8868\u8fbe\u5f0f\uff0c\u8be5\u51fd\u6570\u7528\u4e8e\u7edf\u8ba1\u5728\u7b26\u5408\u641c\u7d22\u6761\u4ef6\u7684\u8bb0\u5f55\u603b\u6570\uff1b<\/p>\n\n\n\n
count(expr)\u51fd\u6570\u6267\u884c\u6548\u7387\u4ece\u4f4e\u5230\u9ad8\u6392\u5e8f\u4e3a\uff1acount(\u975e\u4e3b\u952e\u5b57\u6bb5) < count(\u4e3b\u952e) < count(1) \u2248 count(*) \uff1b<\/p>\n\n\n\n
\u5bf9\u4e8e count(1) \u548c count(*) \uff0c\u6548\u7387\u76f8\u5f53\uff0c\u5efa\u8bae\u5c3d\u91cf\u4f7f\u7528 count(*)\uff0c\u56e0\u4e3a MySQL \u4f18\u5316\u5668\u4f1a\u9009\u62e9\u6700\u5c0f\u7684\u7d22\u5f15\u6811\u8fdb\u884c\u7edf\u8ba1\uff0c\u628a\u4f18\u5316\u7684\u95ee\u9898\u4ea4\u7ed9 MySQL \u4f18\u5316\u5668\u53bb\u89e3\u51b3\u5c31\u53ef\u4ee5\u4e86\uff1b<\/p>\n\n\n\n
count(\u5217\u5b57\u6bb5) \u53ea\u7edf\u8ba1\u4e0d\u4e3a NULL \u7684\u603b\u884c\u6570\uff0c\u6bd4\u5982\uff0ccount(name)\uff0c\u5f53name\u5b57\u6bb5\u503c\u4e3aNUll\u65f6\uff0c\u5c31\u4e0d\u4f1a\u88abcount\uff1b\u800c\u5176\u4ed6\u7684count\uff0c\u5b83\u8fd4\u56de\u68c0\u7d22\u5230\u7684\u884c\u6570\uff0c\u65e0\u8bba\u5b83\u4eec\u662f\u5426\u5305\u542b NULL\u503c\uff1b<\/p>\n\n\n\n
count(NULL) \u603b\u662f\u8fd4\u56de 0\uff1b<\/p>\n\n\n\n
count(expr) \u805a\u5408\u51fd\u6570\u5728\u7edf\u8ba1\u65f6\uff0c\u5e26\u4e0e\u4e0d\u5e26 where \u6761\u4ef6\u5b9e\u73b0\u539f\u7406\u5177\u6709\u5dee\u5f02\u6027\uff0c\u6027\u80fd\u4e5f\u5b58\u5728\u5dee\u5f02\u6027\uff1b<\/p>\n\n\n\n
\u5728\u751f\u4ea7\u4e2d\uff0c\u5bf9\u4e8eInnoDB \u5f15\u64ce\uff0c\u5982\u679c\u5bf9\u6570\u636e\u603b\u91cf\u4e0d\u8981\u6c42\u7279\u522b\u7cbe\u786e\uff0c\u53ef\u4ee5\u4f7f\u7528 “show table status” \u65b9\u5f0f\u83b7\u53d6\u603b\u884c\u6570\uff1b<\/p>\n\n\n\n
\u56e0\u4e3a MySQL\u4e00\u76f4\u5728\u5feb\u901f\u53d1\u5c55\uff0c\u6240\u4ee5\u4e0d\u540c\u7684\u7248\u672c\u5b9e\u73b0\u53ef\u80fd\u5b58\u5728\u5dee\u5f02\uff0c\u6240\u4ee5\u5728\u7814\u7a76\u6280\u672f\u7684\u65f6\u5019\u4e00\u5b9a\u8981\u6ce8\u610f\u7248\u672c\u5dee\u5f02\uff1b<\/p>\n\n\n\n
\u751f\u4ea7\u5982\u4f55\u9009\u62e9\u5efa\u8bae\uff1a<\/p>\n\n\n\n
\u5728\u751f\u4ea7\u73af\u5883\u4e2d\uff0c\u5e94\u5c3d\u91cf\u907f\u514d count(expr)\u8fd9\u79cd\u8017\u65f6\u64cd\u4f5c\uff0c\u5982\u679c\u4e00\u5b9a\u8981\u8fdb\u884c\u7edf\u8ba1\uff0c\u53ef\u4ee5\u6839\u636e\u7edf\u8ba1\u6570\u636e\u7684\u7cbe\u786e\u5ea6\u6765\u533a\u5206\u91c7\u7528\u4ec0\u4e48\u65b9\u5f0f\u7edf\u8ba1\uff1b\u4e5f\u53ef\u4ee5\u5728\u4e13\u4e1a DBA \u7684\u6307\u5bfc\u4e0b\u8fdb\u884c\uff0c\u6216\u8005\u901a\u8fc7 explain \u6267\u884c\u8ba1\u5212\u5b8f\u89c2\u770b\u4e0bcount(expr) \u64cd\u4f5c\u4f1a\u626b\u63cf\u591a\u5c11\u6570\u636e\u884c\uff0c\u5982\u679c\u5bf9\u6027\u80fd\u5f71\u54cd\u6bd4\u8f83\u5927\uff0c\u53ef\u4ee5\u9009\u62e9\u5728\u79bb\u7ebf\u5e93\u6216\u8005\u53ea\u8bfb\u5e93\u4e2d\u8fdb\u884c\uff1b<\/p>\n","protected":false},"excerpt":{"rendered":"
\u8f6c\u8f7d\uff1acount(*) vs count(1) \u5982\u4f55\u9009\uff1f \u5728\u5b9e\u9645\u5f00\u53d1\u5de5\u4f5c\u4e2d\uff0c\u96be\u514d\u4f1a\u4f7f\u7528\u5230 MySQL \u7684 co […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[23],"tags":[],"_links":{"self":[{"href":"https:\/\/92it.top\/index.php?rest_route=\/wp\/v2\/posts\/15682"}],"collection":[{"href":"https:\/\/92it.top\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/92it.top\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/92it.top\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/92it.top\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=15682"}],"version-history":[{"count":5,"href":"https:\/\/92it.top\/index.php?rest_route=\/wp\/v2\/posts\/15682\/revisions"}],"predecessor-version":[{"id":15723,"href":"https:\/\/92it.top\/index.php?rest_route=\/wp\/v2\/posts\/15682\/revisions\/15723"}],"wp:attachment":[{"href":"https:\/\/92it.top\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=15682"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/92it.top\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=15682"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/92it.top\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=15682"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}