https://blog.youkuaiyun.com/m0_74903429zh-cnhttps://blog.youkuaiyun.com/https://blog.youkuaiyun.com/m0_74903429/article/details/153470429https://blog.youkuaiyun.com/m0_74903429/article/details/153470429m0_74903429Fri, 17 Oct 2025 15:12:49 +0800https://blog.youkuaiyun.com/m0_74903429/article/details/153344072https://blog.youkuaiyun.com/m0_74903429/article/details/153344072m0_74903429Wed, 15 Oct 2025 20:38:39 +08008 (S2）)相当于 select sname, rating from S2 where rating>8;distinct， 表示选择哪些列。如：σ rating>8 (S2）相当于 select * from S2 where rating>8;（x， cross-product）相当于SQL语句中的from，表示穷举所有集合两边元素的组合量。（σ， selection）相当于SQL语句中的where，表示选出满足一定条件的行。]]>https://blog.youkuaiyun.com/m0_74903429/article/details/130711999https://blog.youkuaiyun.com/m0_74903429/article/details/130711999m0_74903429Tue, 16 May 2023 19:10:17 +0800https://blog.youkuaiyun.com/m0_74903429/article/details/130278587https://blog.youkuaiyun.com/m0_74903429/article/details/130278587m0_74903429Fri, 21 Apr 2023 00:22:59 +0800https://blog.youkuaiyun.com/m0_74903429/article/details/130231774https://blog.youkuaiyun.com/m0_74903429/article/details/130231774m0_74903429Tue, 18 Apr 2023 21:30:34 +0800https://blog.youkuaiyun.com/m0_74903429/article/details/130208578https://blog.youkuaiyun.com/m0_74903429/article/details/130208578m0_74903429Tue, 18 Apr 2023 10:56:22 +0800https://blog.youkuaiyun.com/m0_74903429/article/details/130109941https://blog.youkuaiyun.com/m0_74903429/article/details/130109941m0_74903429Wed, 12 Apr 2023 18:17:26 +0800https://blog.youkuaiyun.com/m0_74903429/article/details/129936042https://blog.youkuaiyun.com/m0_74903429/article/details/129936042m0_74903429Mon, 03 Apr 2023 18:15:23 +0800https://blog.youkuaiyun.com/m0_74903429/article/details/129852055https://blog.youkuaiyun.com/m0_74903429/article/details/129852055m0_74903429Thu, 30 Mar 2023 10:13:28 +0800https://blog.youkuaiyun.com/m0_74903429/article/details/129657966https://blog.youkuaiyun.com/m0_74903429/article/details/129657966m0_74903429Sun, 19 Mar 2023 22:36:47 +0800https://blog.youkuaiyun.com/m0_74903429/article/details/129417791https://blog.youkuaiyun.com/m0_74903429/article/details/129417791m0_74903429Thu, 09 Mar 2023 10:26:02 +0800https://blog.youkuaiyun.com/m0_74903429/article/details/129389062https://blog.youkuaiyun.com/m0_74903429/article/details/129389062m0_74903429Tue, 07 Mar 2023 19:15:06 +0800https://blog.youkuaiyun.com/m0_74903429/article/details/129385857https://blog.youkuaiyun.com/m0_74903429/article/details/129385857m0_74903429Tue, 07 Mar 2023 17:17:20 +0800https://blog.youkuaiyun.com/m0_74903429/article/details/129371222https://blog.youkuaiyun.com/m0_74903429/article/details/129371222m0_74903429Tue, 07 Mar 2023 16:48:22 +0800https://blog.youkuaiyun.com/m0_74903429/article/details/129365436https://blog.youkuaiyun.com/m0_74903429/article/details/129365436m0_74903429Mon, 06 Mar 2023 20:27:35 +0800https://blog.youkuaiyun.com/m0_74903429/article/details/128906666https://blog.youkuaiyun.com/m0_74903429/article/details/128906666m0_74903429Fri, 03 Mar 2023 21:58:22 +0800https://blog.youkuaiyun.com/m0_74903429/article/details/129263011https://blog.youkuaiyun.com/m0_74903429/article/details/129263011m0_74903429Tue, 28 Feb 2023 15:35:26 +0800https://blog.youkuaiyun.com/m0_74903429/article/details/129248666https://blog.youkuaiyun.com/m0_74903429/article/details/129248666m0_74903429Mon, 27 Feb 2023 21:21:12 +0800https://blog.youkuaiyun.com/m0_74903429/article/details/129247186https://blog.youkuaiyun.com/m0_74903429/article/details/129247186m0_74903429Mon, 27 Feb 2023 18:41:53 +0800https://blog.youkuaiyun.com/m0_74903429/article/details/129184334https://blog.youkuaiyun.com/m0_74903429/article/details/129184334m0_74903429Fri, 24 Feb 2023 14:54:25 +0800