{"id":373,"date":"2013-10-23T11:27:46","date_gmt":"2013-10-23T03:27:46","guid":{"rendered":"http:\/\/www.cnmiss.cn\/?p=373"},"modified":"2013-10-23T11:36:49","modified_gmt":"2013-10-23T03:36:49","slug":"mysql%e4%bc%98%e5%8c%96group-by%ef%bc%8d%e6%9d%be%e6%95%a3%e7%b4%a2%e5%bc%95%e6%89%ab%e6%8f%8f%e4%b8%8e%e7%b4%a7%e5%87%91%e7%b4%a2%e5%bc%95%e6%89%ab%e6%8f%8f","status":"publish","type":"post","link":"https:\/\/www.cnmiss.cn\/?p=373","title":{"rendered":"MySQL\u4f18\u5316GROUP BY\uff0d\u677e\u6563\u7d22\u5f15\u626b\u63cf\u4e0e\u7d27\u51d1\u7d22\u5f15\u626b\u63cf"},"content":{"rendered":"
\n

MySQL\u4f18\u5316GROUP BY\uff0d\u677e\u6563\u7d22\u5f15\u626b\u63cf\u4e0e\u7d27\u51d1\u7d22\u5f15\u626b\u63cf<\/h3>\n<\/div>\n
\u00a0\u00a0\u00a0\u00a0\u00a0 \u6ee1\u8db3GROUP BY\u5b50\u53e5\u7684\u6700\u4e00\u822c\u7684\u65b9\u6cd5\u662f\u626b\u63cf\u6574\u4e2a\u8868\u5e76\u521b\u5efa\u4e00\u4e2a\u65b0\u7684\u4e34\u65f6\u8868\uff0c\u8868\u4e2d\u6bcf\u4e2a\u7ec4\u7684\u6240\u6709\u884c\u5e94\u4e3a\u8fde\u7eed\u7684\uff0c\u7136\u540e\u4f7f\u7528\u8be5\u4e34\u65f6\u8868\u6765\u627e\u5230\u7ec4\u5e76\u5e94\u7528\u7d2f\u79ef\u51fd\u6570(\u5982\u679c\u6709)\u3002\u5728\u67d0\u4e9b\u60c5\u51b5\u4e2d\uff0cMySQL\u80fd\u591f\u505a\u5f97\u66f4\u597d\uff0c\u5373\u901a\u8fc7\u7d22\u5f15\u8bbf\u95ee\u800c\u4e0d\u7528\u521b\u5efa\u4e34\u65f6\u8868\u3002<\/div>\n
\u00a0\u00a0\u00a0\u00a0\u00a0 \u4e3aGROUP BY\u4f7f\u7528\u7d22\u5f15\u7684\u6700\u91cd\u8981\u7684\u524d\u63d0\u6761\u4ef6\u662f\u6240\u6709GROUP BY\u5217\u5f15\u7528\u540c\u4e00\u7d22\u5f15\u7684\u5c5e\u6027\uff0c\u5e76\u4e14\u7d22\u5f15\u6309\u987a\u5e8f\u4fdd\u5b58\u5176\u5173\u952e\u5b57\u3002\u662f\u5426\u7528\u7d22\u5f15\u8bbf\u95ee\u6765\u4ee3\u66ff\u4e34\u65f6\u8868\u7684\u4f7f\u7528\u8fd8\u53d6\u51b3\u4e8e\u5728\u67e5\u8be2\u4e2d\u4f7f\u7528\u4e86\u54ea\u90e8\u5206\u7d22\u5f15\u3001\u4e3a\u8be5\u90e8\u5206\u6307\u5b9a\u7684\u6761\u4ef6\uff0c\u4ee5\u53ca\u9009\u62e9\u7684\u7d2f\u79ef\u51fd\u6570\u3002<\/div>\n
\n

\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u7531\u4e8eGROUP BY \u5b9e\u9645\u4e0a\u4e5f\u540c\u6837\u4f1a\u8fdb\u884c\u6392\u5e8f\u64cd\u4f5c\uff0c\u800c\u4e14\u4e0eORDER BY \u76f8\u6bd4\uff0cGROUP BY \u4e3b\u8981\u53ea\u662f\u591a\u4e86\u6392\u5e8f\u4e4b\u540e\u7684\u5206\u7ec4\u64cd\u4f5c\u3002\u5f53\u7136\uff0c\u5982\u679c\u5728\u5206\u7ec4\u7684\u65f6\u5019\u8fd8\u4f7f\u7528\u4e86\u5176\u4ed6\u7684\u4e00\u4e9b\u805a\u5408\u51fd\u6570\uff0c\u90a3\u4e48\u8fd8\u9700\u8981\u4e00\u4e9b\u805a\u5408\u51fd\u6570\u7684\u8ba1\u7b97\u3002\u6240\u4ee5\uff0c\u5728GROUP BY \u7684\u5b9e\u73b0\u8fc7\u7a0b\u4e2d\uff0c\u4e0e ORDER BY \u4e00\u6837\u4e5f\u53ef\u4ee5\u5229\u7528\u5230\u7d22\u5f15\u3002\u5728MySQL \u4e2d\uff0cGROUP BY \u7684\u5b9e\u73b0\u540c\u6837\u6709\u591a\u79cd\uff08\u4e09\u79cd\uff09\u65b9\u5f0f\uff0c\u5176\u4e2d\u6709\u4e24\u79cd\u65b9\u5f0f\u4f1a\u5229\u7528\u73b0\u6709\u7684\u7d22\u5f15\u4fe1\u606f\u6765\u5b8c\u6210 GROUP BY\uff0c\u53e6\u5916\u4e00\u79cd\u4e3a\u5b8c\u5168\u65e0\u6cd5\u4f7f\u7528\u7d22\u5f15\u7684\u573a\u666f\u4e0b\u4f7f\u7528\u3002\u4e0b\u9762\u6211\u4eec\u5206\u522b\u9488\u5bf9\u8fd9\u4e09\u79cd\u5b9e\u73b0\u65b9\u5f0f\u505a\u4e00\u4e2a\u5206\u6790\u3002<\/p>\n

1\u3001\u4f7f\u7528\u677e\u6563\u7d22\u5f15\u626b\u63cf\uff08Loose index scan\uff09\u5b9e\u73b0 GROUP BY<\/h3>\n

\u5bf9\u201c\u677e\u6563\u7d22\u5f15\u626b\u63cf\u201d\u7684\u5b9a\u4e49\uff0c\u672c\u4eba\u770b\u4e86\u5f88\u591a\u7f51\u4e0a\u7684\u4ecb\u7ecd\uff0c\u90fd\u4e0d\u751a\u660e\u767d\u3002\u5728\u6b64\u903b\u5217\u5982\u4e0b\uff1a<\/p>\n

\u5b9a\u4e491\uff1a\u677e\u6563\u7d22\u5f15\u626b\u63cf\uff0c\u5b9e\u9645\u4e0a\u5c31\u662f\u5f53 MySQL \u5b8c\u5168\u5229\u7528\u7d22\u5f15\u626b\u63cf\u6765\u5b9e\u73b0 GROUP BY \u7684\u65f6\u5019\uff0c\u5e76\u4e0d\u9700\u8981\u626b\u63cf\u6240\u6709\u6ee1\u8db3\u6761\u4ef6\u7684\u7d22\u5f15\u952e\u5373\u53ef\u5b8c\u6210\u64cd\u4f5c\u5f97\u51fa\u7ed3\u679c\u3002<\/p>\n

\u5b9a\u4e492\uff1a\u4f18\u5316Group By\u6700\u6709\u6548\u7684\u529e\u6cd5\u662f\u5f53\u53ef\u4ee5\u76f4\u63a5\u4f7f\u7528\u7d22\u5f15\u6765\u5b8c\u5168\u83b7\u53d6\u9700\u8981group\u7684\u5b57\u6bb5\u3002\u4f7f\u7528\u8fd9\u4e2a\u8bbf\u95ee\u65b9\u6cd5\u65f6\uff0cMySQL\u4f7f\u7528\u5bf9\u5173\u952e\u5b57\u6392\u5e8f\u7684\u7d22\u5f15\u7684\u7c7b\u578b\uff08\u6bd4\u5982BTREE\u7d22\u5f15\uff09\u3002\u8fd9\u4f7f\u5f97\u7d22\u5f15\u4e2d\u7528\u4e8egroup\u7684\u5b57\u6bb5\u4e0d\u5fc5\u5b8c\u5168\u6db5\u76d6WHERE\u6761\u4ef6\u4e2d\u7d22\u5f15\u5bf9\u5e94\u7684key\u3002\u7531\u4e8e\u53ea\u5305\u542b\u7d22\u5f15\u4e2d\u5173\u952e\u5b57\u7684\u4e00\u90e8\u5206\uff0c\u56e0\u6b64\u79f0\u4e3a\u677e\u6563\u7684\u7d22\u5f15\u626b\u63cf\u3002
\n\u610f\u601d\u662f\u7d22\u5f15\u4e2d\u7528\u4e8egroup\u7684\u5b57\u6bb5\uff0c\u6ca1\u5fc5\u8981\u5305\u542b\u591a\u5217\u7d22\u5f15\u7684\u5168\u90e8\u5b57\u6bb5\u3002\u4f8b\u5982\uff1a\u6709\u4e00\u4e2a\u7d22\u5f15idx(c1,c2,c3)\uff0c\u90a3\u4e48group by c1\u3001group by c1,c2\u8fd9\u6837c1\u6216c1\u3001c2\u90fd\u53ea\u662f\u7d22\u5f15idx\u7684\u4e00\u90e8\u5206\u3002\u8981\u6ce8\u610f\u7684\u662f\uff0c\u7d22\u5f15\u4e2d\u7528\u4e8egroup\u7684\u5b57\u6bb5\u5fc5\u987b\u7b26\u5408\u7d22\u5f15\u7684\u201c\u6700\u5de6\u524d\u7f00\u201d\u539f\u5219\u3002group by c1,c3\u662f\u4e0d\u4f1a\u4f7f\u7528\u677e\u6563\u7684\u7d22\u5f15\u626b\u63cf\u7684<\/p>\n

\u4f8b\u5982\uff1a<\/p>\n

explain<\/p>\n

\r\nSELECT group_id,gmt_create\r\nFROM group_message\r\nWHERE user_id>1\r\nGROUP BY group_id,gmt_create;<\/pre>\n
\u672c\u4eba\u7406\u89e3\u201c\u5b9a\u4e492\u201d\u7684\u4f8b\u5b50\u8bf4\u660e<\/p>\n
\u6709\u4e00\u4e2a\u7d22\u5f15idx(c1,c2,c3)<\/p>\n
SELECT c1, c2 FROM t1 WHERE c1 < const GROUP BY c1, c2;<\/pre>\n
\u7d22\u5f15\u4e2d\u7528\u4e8egroup\u7684\u5b57\u6bb5\u4e3ac1,c2
\n\u4e0d\u5fc5\u5b8c\u5168\u6db5\u76d6WHERE\u6761\u4ef6\u4e2d\u7d22\u5f15\u5bf9\u5e94\u7684key\uff08where\u6761\u4ef6\u4e2d\u7d22\u5f15\uff0c\u5373\u4e3ac1\uff1bc1\u5bf9\u5e94\u7684key\uff0c\u5373\u4e3aidx\uff09
\n\u7d22\u5f15\u4e2d\u7528\u4e8egroup\u7684\u5b57\u6bb5(c1,c2)\u53ea\u5305\u542b\u7d22\u5f15\u4e2d\u5173\u952e\u5b57(c1,c2,c3)\u7684\u4e00\u90e8\u5206\uff0c\u56e0\u6b64\u79f0\u4e3a\u677e\u6563\u7684\u7d22\u5f15\u626b\u63cf\u3002<\/p>\n
\u8981\u5229\u7528\u5230\u677e\u6563\u7d22\u5f15\u626b\u63cf\u5b9e\u73b0GROUP BY\uff0c\u9700\u8981\u81f3\u5c11\u6ee1\u8db3\u4ee5\u4e0b\u51e0\u4e2a\u6761\u4ef6\uff1a<\/p>\n
\u25c6 \u67e5\u8be2\u9488\u5bf9\u4e00\u4e2a\u5355\u8868<\/p>\n
\u25c6 GROUP BY \u6761\u4ef6\u5b57\u6bb5\u5fc5\u987b\u5728\u540c\u4e00\u4e2a\u7d22\u5f15\u4e2d\u6700\u524d\u9762\u7684\u8fde\u7eed\u4f4d\u7f6e\uff1b
\nGROUP BY\u5305\u62ec\u7d22\u5f15\u7684\u7b2c1\u4e2a\u8fde\u7eed\u90e8\u5206(\u5982\u679c\u5bf9\u4e8eGROUP BY\uff0c\u67e5\u8be2\u6709\u4e00\u4e2aDISTINCT\u5b50\u53e5\uff0c\u5219\u6240\u6709DISTINCT\u7684\u5c5e\u6027\u6307\u5411\u7d22\u5f15\u5f00\u5934)\u3002<\/p>\n
\u25c6 \u5728\u4f7f\u7528GROUP BY \u7684\u540c\u65f6\uff0c\u5982\u679c\u6709\u805a\u5408\u51fd\u6570\uff0c\u53ea\u80fd\u4f7f\u7528 MAX \u548c MIN \u8fd9\u4e24\u4e2a\u805a\u5408\u51fd\u6570\uff0c\u5e76\u4e14\u5b83\u4eec\u5747\u6307\u5411\u76f8\u540c\u7684\u5217\u3002<\/p>\n
\u25c6 \u5982\u679c\u5f15\u7528(where\u6761\u4ef6\u4e2d)\u5230\u4e86\u8be5\u7d22\u5f15\u4e2dGROUP BY \u6761\u4ef6\u4e4b\u5916\u7684\u5b57\u6bb5\u6761\u4ef6\u7684\u65f6\u5019\uff0c\u5fc5\u987b\u4ee5\u5e38\u91cf\u5f62\u5f0f\u5b58\u5728\uff0c\u4f46MIN()\u6216MAX() \u51fd\u6570\u7684\u53c2\u6570\u4f8b\u5916\uff1b
\n\u6216\u8005\u8bf4\uff1a\u7d22\u5f15\u7684\u4efb\u4f55\u5176\u5b83\u90e8\u5206\uff08\u9664\u4e86\u90a3\u4e9b\u6765\u81ea\u67e5\u8be2\u4e2d\u5f15\u7528\u7684GROUP BY\uff09\u5fc5\u987b\u4e3a\u5e38\u6570(\u4e5f\u5c31\u662f\u8bf4\uff0c\u5fc5\u987b\u6309\u5e38\u91cf\u6570\u91cf\u6765\u5f15\u7528\u5b83\u4eec)\uff0c\u4f46MIN()\u6216MAX() \u51fd\u6570\u7684\u53c2\u6570\u4f8b\u5916\u3002<\/p>\n
\u8865\u5145\uff1a\u5982\u679csql\u4e2d\u6709where\u8bed\u53e5\uff0c\u4e14select\u4e2d\u5f15\u7528\u4e86\u8be5\u7d22\u5f15\u4e2dGROUP BY \u6761\u4ef6\u4e4b\u5916\u7684\u5b57\u6bb5\u6761\u4ef6\u7684\u65f6\u5019\uff0cwhere\u4e2d\u8fd9\u4e9b\u5b57\u6bb5\u8981\u4ee5\u5e38\u91cf\u5f62\u5f0f\u5b58\u5728\u3002<\/p>\n
\u25c6 \u5982\u679c\u67e5\u8be2\u4e2d\u6709where\u6761\u4ef6\uff0c\u5219\u6761\u4ef6\u5fc5\u987b\u4e3a\u7d22\u5f15\uff0c\u4e0d\u80fd\u5305\u542b\u975e\u7d22\u5f15\u7684\u5b57\u6bb5<\/p>\n
\u677e\u6563\u7d22\u5f15\u626b\u63cf
\nexplain<\/p>\n
SELECT group_id,user_id\r\nFROM group_message\r\nWHERE group_id between 1 and 4\r\nGROUP BY group_id,user_id;<\/pre>\n
\u677e\u6563\u7d22\u5f15\u626b\u63cf
\nexplain<\/p>\n
SELECT group_id,user_id\r\nFROM group_message\r\nWHERE user_id>1 and group_id=1\r\nGROUP BY group_id,user_id;<\/pre>\n
\u975e\u677e\u6563\u7d22\u5f15\u626b\u63cf
\nexplain<\/p>\n
SELECT group_id,user_id\r\nFROM group_message\r\nWHERE abc=1\r\nGROUP BY group_id,user_id;<\/pre>\n
\u975e\u677e\u6563\u7d22\u5f15\u626b\u63cf
\nexplain<\/p>\n
SELECT group_id,user_id\r\nFROM group_message\r\nWHERE user_id>1 and abc=1\r\nGROUP BY group_id,user_id;<\/pre>\n
\u677e\u6563\u7d22\u5f15\u626b\u63cf\uff0c\u6b64\u7c7b\u67e5\u8be2\u7684EXPLAIN\u8f93\u51fa\u663e\u793aExtra\u5217\u7684Using index for group-by<\/p>\n
\u4e0b\u9762\u7684\u67e5\u8be2\u63d0\u4f9b\u8be5\u7c7b\u7684\u51e0\u4e2a\u4f8b\u5b50\uff0c\u5047\u5b9a\u8868t1(c1,c2,c3,c4)\u6709\u4e00\u4e2a\u7d22\u5f15idx(c1\uff0cc2\uff0cc3)\uff1a<\/p>\n
SELECT c1, c2 FROM t1 GROUP BY c1, c2;\r\n\r\nSELECT DISTINCT c1, c2 FROM t1;\r\n\r\nSELECT c1, MIN(c2) FROM t1 GROUP BY c1;\r\n\r\nSELECT c1, c2 FROM t1 WHERE c1 < const GROUP BY c1, c2;\r\n\r\nSELECT MAX(c3), MIN(c3), c1, c2 FROM t1 WHERE c2 > const GROUP BY c1, c2;\r\n\r\nSELECT c2 FROM t1 WHERE c1 < const GROUP BY c1, c2;\r\n\r\nSELECT c1, c2 FROM t1 WHERE c3 = const GROUP BY c1, c2\uff1b<\/pre>\n
\u7531\u4e8e\u4e0a\u8ff0\u539f\u56e0\uff0c\u4e0d\u80fd\u7528\u8be5\u5feb\u901f\u9009\u62e9\u65b9\u6cd5\u6267\u884c\u4e0b\u9762\u7684\u67e5\u8be2\uff1a<\/p>\n
1\u3001\u9664\u4e86MIN()\u6216MAX()\uff0c\u8fd8\u6709\u5176\u5b83\u7d2f\u79ef\u51fd\u6570\uff0c\u4f8b\u5982\uff1a<\/p>\n
SELECT c1, SUM(c2) FROM t1 GROUP BY c1;<\/pre>\n
2\u3001GROUP BY\u5b50\u53e5\u4e2d\u7684\u57df\u4e0d\u5f15\u7528\u7d22\u5f15\u5f00\u5934\uff0c\u5982\u4e0b\u6240\u793a\uff1a<\/p>\n
SELECT c1,c2 FROM t1 GROUP BY c2, c3;<\/pre>\n
3\u3001\u67e5\u8be2\u5f15\u7528\u4e86GROUP BY\u90e8\u5206\u540e\u9762\u7684\u5173\u952e\u5b57\u7684\u4e00\u90e8\u5206\uff0c\u5e76\u4e14\u6ca1\u6709\u7b49\u4e8e\u5e38\u91cf\u7684\u7b49\u5f0f\uff0c\u4f8b\u5982\uff1a<\/p>\n
SELECT c1,c3 FROM t1 GROUP BY c1, c2\uff1b<\/pre>\n
\u8fd9\u4e2a\u4f8b\u5b50\u4e2d\uff0c\u5f15\u7528\u5230\u4e86c3\uff08c3\u5fc5\u987b\u4e3a\u7ec4\u5408\u7d22\u5f15\u4e2d\u7684\u4e00\u4e2a\uff09\uff0c\u56e0\u4e3agroup by \u4e2d\u6ca1\u6709c3\u3002\u5e76\u4e14\u6ca1\u6709\u7b49\u4e8e\u5e38\u91cf\u7684\u7b49\u5f0f\u3002\u6240\u4ee5\u4e0d\u80fd\u4f7f\u7528\u677e\u6563\u7d22\u5f15\u626b\u63cf<\/p>\n
\u53ef\u4ee5\u8fd9\u6837\u6539\u4e00\u4e0b\uff1a<\/p>\n
SELECT c1,c3 FROM t1 where c3='a' GROUP BY c1, c2<\/pre>\n
\u4e0b\u9762\u8fd9\u4e2a\u4f8b\u5b50\u4e0d\u80fd\u4f7f\u7528\u677e\u6563\u7d22\u5f15\u626b\u63cf<\/p>\n
SELECT c1,c3 FROM t1 where c3='a' GROUP BY c1, c2<\/pre>\n
\u4e3a\u4ec0\u4e48\u677e\u6563\u7d22\u5f15\u626b\u63cf\u7684\u6548\u7387\u4f1a\u5f88\u9ad8\uff1f<\/p>\n
\u7b54\uff1a\u56e0\u4e3a\u5728\u6ca1\u6709WHERE \u5b50\u53e5\uff0c\u4e5f\u5c31\u662f\u5fc5\u987b\u7ecf\u8fc7\u5168\u7d22\u5f15\u626b\u63cf\u7684\u65f6\u5019\uff0c \u677e\u6563\u7d22\u5f15\u626b\u63cf\u9700\u8981\u8bfb\u53d6\u7684\u952e\u503c\u6570\u91cf\u4e0e\u5206\u7ec4\u7684\u7ec4\u6570\u91cf\u4e00\u6837\u591a\uff0c\u4e5f\u5c31\u662f\u8bf4\u6bd4\u5b9e\u9645\u5b58\u5728\u7684\u952e\u503c\u6570\u76ee\u8981\u5c11\u5f88\u591a\u3002\u800c\u5728WHERE \u5b50\u53e5\u5305\u542b\u8303\u56f4\u5224\u65ad\u5f0f\u6216\u8005\u7b49\u503c\u8868\u8fbe\u5f0f\u7684\u65f6\u5019\uff0c \u677e\u6563\u7d22\u5f15\u626b\u63cf\u67e5\u627e\u6ee1\u8db3\u8303\u56f4\u6761\u4ef6\u7684\u6bcf\u4e2a\u7ec4\u7684\u7b2c1 \u4e2a\u5173\u952e\u5b57\uff0c\u5e76\u4e14\u518d\u6b21\u8bfb\u53d6\u5c3d\u53ef\u80fd\u6700\u5c11\u6570\u91cf\u7684\u5173\u952e\u5b57\u3002<\/p>\n
2\u3001\u4f7f\u7528\u7d27\u51d1\u7d22\u5f15\u626b\u63cf\uff08Tight index scan\uff09\u5b9e\u73b0 GROUP BY<\/h3>\n
\u7d27\u51d1\u7d22\u5f15\u626b\u63cf\u5b9e\u73b0 GROUP BY \u548c\u677e\u6563\u7d22\u5f15\u626b\u63cf\u7684\u533a\u522b\u4e3b\u8981\u5728\u4e8e\uff1a<\/p>\n
\u7d27\u51d1\u7d22\u5f15\u626b\u63cf\u9700\u8981\u5728\u626b\u63cf\u7d22\u5f15\u7684\u65f6\u5019\uff0c\u8bfb\u53d6\u6240\u6709\u6ee1\u8db3\u6761\u4ef6\u7684\u7d22\u5f15\u952e\uff0c\u7136\u540e\u518d\u6839\u636e\u8bfb\u53d6\u51fa\u7684\u6570\u636e\u6765\u5b8c\u6210 GROUP BY \u64cd\u4f5c\u5f97\u5230\u76f8\u5e94\u7ed3\u679c\u3002<\/p>\n
\u8fd9\u65f6\u5019\u7684\u6267\u884c\u8ba1\u5212\u7684 Extra \u4fe1\u606f\u4e2d\u5df2\u7ecf\u6ca1\u6709\u201cUsing index for group-by\u201d\u4e86\uff0c\u4f46\u5e76\u4e0d\u662f\u8bf4 MySQL \u7684 GROUP BY \u64cd\u4f5c\u5e76\u4e0d\u662f\u901a\u8fc7\u7d22\u5f15\u5b8c\u6210\u7684\uff0c\u53ea\u4e0d\u8fc7\u662f\u9700\u8981\u8bbf\u95ee WHERE \u6761\u4ef6\u6240\u9650\u5b9a\u7684\u6240\u6709\u7d22\u5f15\u952e\u4fe1\u606f\u4e4b\u540e\u624d\u80fd\u5f97\u51fa\u7ed3\u679c\u3002\u8fd9\u5c31\u662f\u901a\u8fc7\u7d27\u51d1\u7d22\u5f15\u626b\u63cf\u6765\u5b9e\u73b0 GROUP BY \u7684\u6267\u884c\u8ba1\u5212\u8f93\u51fa\u4fe1\u606f\u3002<\/p>\n
\u5728 MySQL \u4e2d\uff0cMySQL Query Optimizer \u9996\u5148\u4f1a\u9009\u62e9\u5c1d\u8bd5\u901a\u8fc7\u677e\u6563\u7d22\u5f15\u626b\u63cf\u6765\u5b9e\u73b0 GROUP BY \u64cd\u4f5c\uff0c\u5f53\u53d1\u73b0\u67d0\u4e9b\u60c5\u51b5\u65e0\u6cd5\u6ee1\u8db3\u677e\u6563\u7d22\u5f15\u626b\u63cf\u5b9e\u73b0 GROUP BY \u7684\u8981\u6c42\u4e4b\u540e\uff0c\u624d\u4f1a\u5c1d\u8bd5\u901a\u8fc7\u7d27\u51d1\u7d22\u5f15\u626b\u63cf\u6765\u5b9e\u73b0\u3002<\/p>\n
\u5f53 GROUP BY \u6761\u4ef6\u5b57\u6bb5\u5e76\u4e0d\u8fde\u7eed\u6216\u8005\u4e0d\u662f\u7d22\u5f15\u524d\u7f00\u90e8\u5206\u7684\u65f6\u5019\uff0cMySQL Query Optimizer \u65e0\u6cd5\u4f7f\u7528\u677e\u6563\u7d22\u5f15\u626b\u63cf\u3002<\/p>\n
\u8fd9\u65f6\u68c0\u67e5where \u4e2d\u7684\u6761\u4ef6\u5b57\u6bb5\u662f\u5426\u6709\u7d22\u5f15\u7684\u524d\u7f00\u90e8\u5206\uff0c\u5982\u679c\u6709\u6b64\u524d\u7f00\u90e8\u5206\uff0c\u4e14\u8be5\u90e8\u5206\u662f\u4e00\u4e2a\u5e38\u91cf\uff0c\u4e14\u4e0egroup by \u540e\u7684\u5b57\u6bb5\u7ec4\u5408\u8d77\u6765\u6210\u4e3a\u4e00\u4e2a\u8fde\u7eed\u7684\u7d22\u5f15\u3002\u8fd9\u65f6\u6309\u7d27\u51d1\u7d22\u5f15\u626b\u63cf\u3002<\/p>\n
SELECT max(gmt_create)\r\nFROM group_message\r\nWHERE group_id = 2\r\nGROUP BY user_id<\/pre>\n
\u9700\u8bfb\u53d6group_id=2\u7684\u6240\u6709\u6570\u636e\uff0c\u7136\u540e\u5728\u8bfb\u53d6\u7684\u6570\u636e\u4e2d\u5b8c\u6210group by\u64cd\u4f5c\u5f97\u5230\u7ed3\u679c\u3002\uff08\u8fd9\u91ccgroup by \u5b57\u6bb5\u5e76\u4e0d\u662f\u4e00\u4e2a\u8fde\u7eed\u7d22\u5f15\uff0c\u6b63\u597dwhere \u4e2dgroup_id\u6b63\u597d\u5f25\u8865\u7f3a\u5931\u7684\u7d22\u5f15\u952e\uff0c\u53c8\u6070\u597d\u662f\u4e00\u4e2a\u5e38\u91cf\uff0c\u56e0\u6b64\u4f7f\u7528\u7d27\u51d1\u7d22\u5f15\u626b\u63cf\uff09<\/p>\n
group_id user_id \u8fd9\u4e2a\u987a\u5e8f\u662f\u53ef\u4ee5\u4f7f\u7528\u8be5\u7d22\u5f15\u3002\u5982\u679c\u8fde\u63a5\u7684\u987a\u5e8f\u4e0d\u7b26\u5408\u7d22\u5f15\u7684\u201c\u6700\u5de6\u524d\u7f00\u201d\u539f\u5219\uff0c\u5219\u4e0d\u4f7f\u7528\u7d27\u51d1\u7d22\u5f15\u626b\u63cf\u3002<\/p>\n
\u4ee5\u4e0b\u4f8b\u5b50\u4f7f\u7528\u7d27\u51d1\u7d22\u5f15\u626b\u63cf<\/p>\n
GROUP BY\u4e2d\u6709\u4e00\u4e2a\u5dee\u8ddd\uff0c\u4f46\u5df2\u7ecf\u7531\u6761\u4ef6user_id = 1\u8986\u76d6\u3002<\/p>\n
explain<\/p>\n
SELECT group_id,gmt_create\r\nFROM group_message\r\nWHERE user_id = 1 GROUP BY group_id,gmt_create<\/pre>\n
GROUP BY\u4e0d\u4ee5\u5173\u952e\u5b57\u7684\u7b2c1\u4e2a\u5143\u7d20\u5f00\u59cb\uff0c\u4f46\u662f\u6709\u4e00\u4e2a\u6761\u4ef6\u63d0\u4f9b\u8be5\u5143\u7d20\u7684\u5e38\u91cf
\nexplain<\/p>\n
SELECT group_id,gmt_create\r\nFROM group_message\r\nWHERE group_id = 1 GROUP BY user_id,gmt_create<\/pre>\n
\u4e0b\u9762\u7684\u4f8b\u5b50\u90fd\u4e0d\u4f7f\u7528\u7d27\u51d1\u7d22\u5f15\u626b\u63cf<\/p>\n
user_id,gmt_create \u8fde\u63a5\u8d77\u6765\u5e76\u4e0d\u7b26\u5408\u7d22\u5f15\u201c\u6700\u5de6\u524d\u7f00\u201d\u539f\u5219
\nexplain<\/p>\n
SELECT group_id,gmt_create\r\nFROM group_message\r\nWHERE user_id = 1 GROUP BY gmt_create<\/pre>\n
group_id,gmt_create \u8fde\u63a5\u8d77\u6765\u5e76\u4e0d\u7b26\u5408\u7d22\u5f15\u201c\u6700\u5de6\u524d\u7f00\u201d\u539f\u5219<\/p>\n
explain<\/p>\n
SELECT gmt_create\r\nFROM group_message\r\nWHERE group_id=1 GROUP BY gmt_create;<\/pre>\n
\u00a03\u3001\u4f7f\u7528\u4e34\u65f6\u8868\u5b9e\u73b0 GROUP BY<\/h3>\n
MySQL Query Optimizer \u53d1\u73b0\u4ec5\u4ec5\u901a\u8fc7\u7d22\u5f15\u626b\u63cf\u5e76\u4e0d\u80fd\u76f4\u63a5\u5f97\u5230 GROUP BY \u7684\u7ed3\u679c\u4e4b\u540e\uff0c\u4ed6\u5c31\u4e0d\u5f97\u4e0d\u9009\u62e9\u901a\u8fc7\u4f7f\u7528\u4e34\u65f6\u8868\u7136\u540e\u518d\u6392\u5e8f\u7684\u65b9\u5f0f\u6765\u5b9e\u73b0 GROUP BY\u4e86\u3002\u5728\u8fd9\u6837\u793a\u4f8b\u4e2d\u5373\u662f\u8fd9\u6837\u7684\u60c5\u51b5\u3002 group_id \u5e76\u4e0d\u662f\u4e00\u4e2a\u5e38\u91cf\u6761\u4ef6\uff0c\u800c\u662f\u4e00\u4e2a\u8303\u56f4\uff0c\u800c\u4e14 GROUP BY \u5b57\u6bb5\u4e3a user_id\u3002\u6240\u4ee5 MySQL \u65e0\u6cd5\u6839\u636e\u7d22\u5f15\u7684\u987a\u5e8f\u6765\u5e2e\u52a9 GROUP BY \u7684\u5b9e\u73b0\uff0c\u53ea\u80fd\u5148\u901a\u8fc7\u7d22\u5f15\u8303\u56f4\u626b\u63cf\u5f97\u5230\u9700\u8981\u7684\u6570\u636e\uff0c\u7136\u540e\u5c06\u6570\u636e\u5b58\u5165\u4e34\u65f6\u8868\uff0c\u7136\u540e\u518d\u8fdb\u884c\u6392\u5e8f\u548c\u5206\u7ec4\u64cd\u4f5c\u6765\u5b8c\u6210 GROUP BY\u3002<\/p>\n
explain<\/p>\n
SELECT group_id\r\nFROM group_message\r\nWHERE group_id between 1 and 4\r\nGROUP BY user_id;<\/pre>\n
\u793a\u4f8b\u6570\u636e\u5e93\u6587\u4ef6<\/h2>\n
-- --------------------------------------------------------\r\n-- Host: 127.0.0.1\r\n-- Server version: 5.1.57-community - MySQL Community Server (GPL)\r\n-- Server OS: Win32\r\n-- HeidiSQL version: 7.0.0.4156\r\n-- Date\/time: 2012-08-20 16:52:10\r\n-- --------------------------------------------------------\r\n\r\n\/*!40101 SET @OLD_CHARACTER_SET_CLIENT=@@CHARACTER_SET_CLIENT *\/;\r\n\/*!40101 SET NAMES utf8 *\/;\r\n\/*!40014 SET FOREIGN_KEY_CHECKS=0 *\/;\r\n\r\n-- Dumping structure for table test.group_message\r\nDROP TABLE IF EXISTS `group_message`;\r\nCREATE TABLE IF NOT EXISTS `group_message` (\r\n`id` int(10) unsigned NOT NULL AUTO_INCREMENT,\r\n`group_id` int(10) unsigned DEFAULT NULL,\r\n`user_id` int(10) unsigned DEFAULT NULL,\r\n`gmt_create` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,\r\n`abc` int(11) NOT NULL DEFAULT '0',\r\nPRIMARY KEY (`id`),\r\nKEY `group_id_user_id_gmt_create` (`group_id`,`user_id`,`gmt_create`)\r\n) ENGINE=MyISAM AUTO_INCREMENT=27 DEFAULT CHARSET=utf8;\r\n\r\n-- Dumping data for table test.group_message: 0 rows\r\nDELETE FROM `group_message`;\r\n\/*!40000 ALTER TABLE `group_message` DISABLE KEYS *\/;\r\nINSERT INTO `group_message` (`id`, `group_id`, `user_id`, `gmt_create`, `abc`) VALUES\r\n(1, 1, 1, '2012-08-20 09:25:35', 1),\r\n(2, 2, 1, '2012-08-20 09:25:39', 1),\r\n(3, 2, 2, '2012-08-20 09:25:47', 1),\r\n(4, 3, 1, '2012-08-20 09:25:50', 2),\r\n(5, 3, 2, '2012-08-20 09:25:52', 2),\r\n(6, 3, 3, '2012-08-20 09:25:54', 0),\r\n(7, 4, 1, '2012-08-20 09:25:57', 0),\r\n(8, 4, 2, '2012-08-20 09:26:00', 0),\r\n(9, 4, 3, '2012-08-20 09:26:02', 0),\r\n(10, 4, 4, '2012-08-20 09:26:06', 0),\r\n(11, 5, 1, '2012-08-20 09:26:09', 0),\r\n(12, 5, 2, '2012-08-20 09:26:12', 0),\r\n(13, 5, 3, '2012-08-20 09:26:13', 0),\r\n(14, 5, 4, '2012-08-20 09:26:15', 0),\r\n(15, 5, 5, '2012-08-20 09:26:17', 0),\r\n(16, 6, 1, '2012-08-20 09:26:20', 0),\r\n(17, 7, 1, '2012-08-20 09:26:23', 0),\r\n(18, 7, 2, '2012-08-20 09:26:28', 0),\r\n(19, 8, 1, '2012-08-20 09:26:32', 0),\r\n(20, 8, 2, '2012-08-20 09:26:35', 0),\r\n(21, 9, 1, '2012-08-20 09:26:37', 0),\r\n(22, 9, 2, '2012-08-20 09:26:40', 0),\r\n(23, 10, 1, '2012-08-20 09:26:42', 0),\r\n(24, 10, 2, '2012-08-20 09:26:44', 0),\r\n(25, 10, 3, '2012-08-20 09:26:51', 0),\r\n(26, 11, 1, '2012-08-20 09:26:54', 0);\r\n\/*!40000 ALTER TABLE `group_message` ENABLE KEYS *\/;\r\n\/*!40014 SET FOREIGN_KEY_CHECKS=1 *\/;\r\n\/*!40101 SET CHARACTER_SET_CLIENT=@OLD_CHARACTER_SET_CLIENT *\/;<\/pre>\n
\u53c2\u8003\u6587\u732e<\/h2>\n
1\u3001MySQL\u5982\u4f55\u4f18\u5316GROUP BY\u00a0http:\/\/www.iteedu.com\/\/database\/mysql\/mysqlmanualcn\/optimization\/group-by-optimization.php<\/a>
\n2\u3001\u8be6\u89e3MySQL\u5206\u7ec4\u67e5\u8be2Group By\u5b9e\u73b0\u539f\u7406\u00a0http:\/\/www.51testing.com\/html\/52\/n-229952.html<\/a>
\n3\u3001\u677e\u6563\u7684\u7d22\u5f15\u626b\u63cf(Loose index scan)\u00a0http:\/\/www.coder4.com\/archives\/1344<\/a>
\n4\u3001MySQL\u5b66\u4e60\u7b14\u8bb0\u00a0http:\/\/leeon.me\/a\/mysql-notes-2<\/a><\/div>\n","protected":false},"excerpt":{"rendered":"
MySQL\u4f18\u5316GROUP BY\uff0d\u677e\u6563\u7d22\u5f15\u626b\u63cf\u4e0e\u7d27\u51d1\u7d22\u5f15\u626b\u63cf \u00a0\u00a0\u00a0\u00a0\u00a0 \u6ee1\u8db3GR … \u7ee7\u7eed\u9605\u8bfb →<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[24],"class_list":["post-373","post","type-post","status-publish","format-standard","hentry","category-xxbj","tag-mysql"],"_links":{"self":[{"href":"https:\/\/www.cnmiss.cn\/index.php?rest_route=\/wp\/v2\/posts\/373","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.cnmiss.cn\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.cnmiss.cn\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.cnmiss.cn\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.cnmiss.cn\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=373"}],"version-history":[{"count":0,"href":"https:\/\/www.cnmiss.cn\/index.php?rest_route=\/wp\/v2\/posts\/373\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.cnmiss.cn\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=373"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.cnmiss.cn\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=373"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.cnmiss.cn\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=373"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}