• （1）Here is Beijing,where I grow up.where其实也是指代北京，不过where是在北京的意思，也就是 in Beijing（2）Here is Beijing,in which I grow up.其实which就是指代北京，也就是I grow up in which然后把in ...
（1）Here is Beijing,where I grow up.where其实也是指代北京，不过where是在北京的意思，也就是 in Beijing（2）Here is Beijing,in which I grow up.其实which就是指代北京，也就是I grow up in which然后把in which提到了前面（3）Here is Beijing,from which I come.这句也好懂了吧，就是I come from which，which就是Beijing。（4）Here is my car,by which I go to work.这句就是 I go to work by which，which就是carin from by使用哪个就是根据后半句的语境来决定的
转载于:https://www.cnblogs.com/or2-/p/5187157.html
展开全文
• Linux中which,whereis,locate和find的区别linux中对文件进行查找主要使用的就是标题中的四个命令,为什么会提供四个命令,当然是因为他们彼此功能上存在差异,下面就简单介绍一下这四个命令的特点.whichwhich returns ...
linux中对文件进行查找主要使用的就是标题中的四个命令,为什么会提供四个命令,当然是因为他们彼此功能上存在差异,下面就简单介绍一下这四个命令的特点.

which

which returns the pathnames of the files (or links) which would be executed in the current environment, had its arguments been given as  commands  in a strictly POSIX-conformant shell.  It does this by searching
the PATH for executable files matching the names of the  arguments.  It does not follow symbolic links.

从上面的说明我们可以知道,which只查找当前用户PATH环境变量(可以通过echo $PATH查看)下的可执行文件,这样的话范围就比较窄,当然速度也是飞起. whereis whereis locates the binary, source and manual files for the specified command names. The supplied names are first stripped of leading pathname components and any (single) trailing extension of the form .ext(for example: .c) Prefixes of s. resulting from use of source code control are also dealt with. whereis then attempts to locate the desired program in the standard Linux places, and in the places specified by$PATH and $MANPATH. whereis命令只能用于程序名的搜索，而且只搜索二进制文件（参数-b）、man说明文件（参数-m）和源代码文件（参数-s）。如果省略参数，则返回所有信息。 和find相比，whereis查找的速度非常快，这是因为linux系统会将 系统内的所有文件都记录在一个数据库文件中，当使用whereis和下面即将介绍的locate时，会从数据库中查找数据，而不是像find命令那样，通 过遍历硬盘来查找，效率自然会很高。 但是该数据库文件并不是实时更新，默认情况下时一星期更新一次，因此，我们在用whereis和locate 查找文件时，有时会找到已经被删除的数据，或者刚刚建立文件，却无法查找到，原因就是因为数据库文件没有被更新。 locate locate文件用于查找文件(包括普通文件),他与全盘搜索的find命令的区别是locate依赖于一个已经建好的并且每天进行更新的数据库.这样的话也会存在whereis存在的问题,就是数据库可能不是最新的,会缺少文件或者存在已经被删除的文件.但是它带来的确实速度上的大幅度提升,大家自己去感受一下就知道了.locate支持正则表达式匹配 当然我们也可以手动去更新这个数据库,只要在任意目录下运作sudo updatedb命令即可. find 这个命令就像我们在windows下面的搜索了,进行的是在指定目录下的扫描,速度会稍微慢一点. find可用的参数非常多,支持正则表达式匹配. 如果我们不知道要找的文件在那个目录下,那就用根目录/去找吧,如: find / -name filename -print 上面的命令意思是在全局中查找名字为filename的文件或者文件名,找到之后print出来 展开全文 • 1：where和If最本质的区别...1.1：The WHERE statement examines what is in the input page buffer and selectsobservations before they are loaded in the program data vector, which results in asavings in C... 1：where和If最本质的区别，以及一些小的区别 1.1：The WHERE statement examines what is in the input page buffer and selects observations before they are loaded in the program data vector, which results in a savings in CPU operations(Where从buffer中进行筛选再读入pdv) The subsetting IF statement loads all observations sequentially into the program data vector. If the statement finds a match and the statement is true, then the data is processed and is written to the output page buffer(If先读入pdv再进行筛选) 1.2：if可以从input的数据和sas数据集的数据中进行筛选，where只能筛选sas数据集的数据 if可以if语句的条件条件选择子句，where不能 where比if高效 if语句<可执行语句> IF statement tells SAS which observations to include, the DELETE statement tells SAS which observations to exclude IF Sex = 'f'; IF Sex = 'm' THEN DELETE; 作用一样！ data b; set sashelp.class; if _n_ le 4; *如果if为真，则继续执行if后面的语句，最后输出满足if的条件的观测，如果if为假则立刻返回到data步开头继续执行下一条set语句; y = 'now'; /* y = 'now'; if _n_ le 4;也能得出同样的结果，但是效率相对来说较低，因为要重复执行y的赋值语句 */ run;if的另外两种格式if x=3 then y=4; 对于要表达的只有一条数据就用thenif x=3 then do y=4;z=5;end; 对于要表达的有多条语句就用then do end; NOTE: 从数据集 SASHELP.CLASS. 读取了 19 个观测NOTE: 数据集 WORK.B 有 4 个观测和 6 个变量。NOTE: “DATA 语句”所用时间（总处理时间）: 实际时间 0.03 秒 CPU 时间 0.03 秒 日志中读入了19个观测，证明是全部读入再一个个判断是否满足条件 where语句(where=选项)<不可执行语句> 是在系统准备把观测读入pdv之前制定数据必须满足的一个条件。 WHERE where-expression-1<logical-operator where-expression-n>; logical-operator can be AND, AND NOT, OR, or OR NOT where表达式的算符：between and、is missing (is null)、contain (？)、like、same and、in TIPS： 1：where语句不能与自动变量连用以及新创建的变量连用，因为where语句在pdv之前执行 2：使用where语句时，必须保证读入数据集的完整性，不能使用firstobs=2等不能完整读入数据集的选项 3：当where选项与where语句同时作用于一个数据集的时候，系统只会考虑where选项，where选项可以只对某一个数据集起作用，而where语句是对所有的数据集起作用。 4：当data步包含where语句和by语句时，where语句先于by语句之前被执行，by组对执行完毕后的数据集重新定义first/last变量。 5：能用contains的地方都能用like、所以首先考虑用like。where x like a_b%; '_'表示正好有一个字符与之匹配，'%'表示可以替代任意多个字符 data a; input x y@@; cards; 1 10 1 20 0 200 2 30 2 40 3 50 3 60 4 70 3 80 4 400 ; run; proc sort data=a;by x;run; data b; set a; *where x ; *后面不添加条件是筛选x不为0和不为缺失值的数值型数据，只适用于数值型; where x is not missing; *筛选x不为缺失值的数据包括0,适用于数值型和字符型; run; proc print data=b noobs; where和if的最重要的几点区别 1：where不可执行、if可执行 2：where有自己特定的表达式，if是是通用表达式 例如where x is missing; 3：where只能从现有的sas数据集中选择观测，if语句还可以用input语句产生的观测中选择。*商用的一般都是现有的sas数据集; 4：where的效率比if高 5：何时使用if何时使用where?如果需要对pdv观测进行处理才能决定哪条观测，只能使用if。其余能使用where 转载于:https://www.cnblogs.com/yican/p/3809555.html 展开全文 • Which statements are true regarding the WHERE and HAVING clauses in a SELECT statement? (Choose all that apply.) A. The HAVING clause can be used with aggregate functions in subqueries. B. Th 一、原题 Which statements are true regarding the WHERE and HAVING clauses in a SELECT statement?(Choose all that apply.)A. The HAVING clause can be used with aggregate functions in subqueries.B. The WHERE clause can be used to exclude rows after dividing them into groups.C. The WHERE clause can be used to exclude rows before dividing them into groups.D. The aggregate functions and columns used in the HAVING clause must be specified in the SELECT list of the query. E. The WHERE and HAVING clauses can be used in the same statement only if they are applied to different columns in the table. 答案：AC 二、题目翻译 关于WHERE和HAVING子句哪句话是正确的(选择所有正确的选项） A.HAVING子句能在子查询中使用聚合函数。 B.WHERE子句能被用于在划分组之后排除行。 C.WHERE子句能被用于在划分组之前排除行。 D.用在HAVING子句中的聚合函数和列必须被指定到SELECT语句的列表中。 E.只要应用不同的列，WHERE和HAVING子句就能用在相同的语句中。 三、题目解析 WHERE是在分组之前筛选数据，HAVING是在分组之后筛选数据。 WHERE子句中不能使用聚合函数，HAVING子句中可以使用聚合函数。  展开全文 • What is Cross Join in SQL?...The SQL CROSS JOIN produces a result set which is the number of rows in the first table multiplied by the number of rows in the second table if no WHERE clause is used along • <p>A SIMD computer consists of n identical processors, each with its own local memory, where it is possible to store data. 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