join

Walrus Joins In1What will Walrus do?Everyone at the North Pole was very excited. There was going to be a show and ANYONE could be in it.‘I will do skating," said Arctic Fox. ‘I"m good at that!"‘I"ll do tumbling," said Polar Bear. ‘No one tumbles quite like me!"‘I"ll do singing," said Seal. ‘Everyone says I have a very fine voice!"‘Then I"ll do diving," said Whale. ‘I won a prize for diving at school, you know!"They all looked at Walrus. ‘What will YOU do?" they asked. But Walrus was not good at anything. He wasn"t good at skating, and he wasn"t good at tumbling.He was terrible at singing, and when he tried to dive, he always got water up his nose. He sat and chewed his whiskers sadly. ‘Never mind," said Arctic Fox. ‘You can watch us."Arctic Fox and Polar Bear and Seal and Whale practised hard for the big show.Walrus hid behind a snowdrift and watched, and chewed his whiskers.He wished he was good at something.2The big nightAt last, the big night arrived. Everyone sat down and waited for the show to begin. Walrus sat in the front row. He was very excited. Fox came onto the ice and bowed. Everyone cheered.Then Fox began to skate. Fox skated forwards and backwards and sideways. She skated in perfect circles and figures of eight. She was elegant and amazing!Walrus watched and he loved what he saw. Fox made it all look so easy.Walrus was sure that if he really tried he could skate just like Fox.He couldn"t stop himself. He just had to leap onto the ice and join in with Fox. “I can skate,” he cried. “Look at me!”But Walrus couldn"t skate at all. He could only trip up and fall over.He bumped into Fox, and Fox went flat on her face. FLOMP!Fox was very upset. “Walrus has RUINED my act,” she wailed. Next, it was Polar Bear"s turn. He rolled out across the ice like a big, white snowball. Everyone clapped wildly.Then Polar Bear began to tumble. He did jumps and spins and somersaults, and stood on his head.Walrus watched and he loved what he saw. Polar Bear made it all look such fun. Walrus was sure that this time, if he really tried, he could tumble just like Polar Bear. All of a sudden, Walrus just couldn"t stop himself, and he leaped onto the ice. “I can tumble too,” he cried. “Look at me!”But Walrus couldn"t tumble at all. He could only trip up and fall over. He tripped up Polar Bear, who came down with a WALLOP!Of course, Polar Bear was pretty angry. “Walrus has RUINED my act,” he wailed. 3From bad to worseIt was Seal"s turn next. She gave Walrus a don"t-you-dare stare, and then she started to sing:“O, how many heart rejoices when I see the Northern Lights.My ear is filled with voices sweetly singing in the night!”Walrus listened. What a beautiful song! Surely if he really tried, he could sing as beautifully as Seal? Oh, dear. Walrus just couldn"t stop himself again.“I know that song,” he cried. “I can sing it too!”He leaped up and started singing along with Seal.But Walrus couldn"t sing! He sounded terrible. In fact, he sounded like a rusty old bucket.Seal stopped singing and burst into floods of tears.“Walrus has RUINED my song,” she wailed.Wale was last. He was pretty certain that Walrus could not ruin his act.Whale leaped high out of the water. Then he fell back with an enormous splash!Walrus watched. He wished that he could dive like that. His flippers began to twitch, and his whiskers bristled with excitement. He tried and tried his very best not to join in.But then he had a brilliant idea.“I"ll hold my nose when I dive,” he thought. “Then the water won"t go up it!”Walrus just couldn"t stop himself. He had to join in.“Everybody, look at me!” he cried, as he leaped into the water. “I can dive too!”But just at that moment, Whale was getting ready to spout a big jet of water.WHOOSH!Whale spouted Walrus high into the air!Everyone was watching Walrus now. They clapped and cheered as Walrus landed back in the sea with a SPLOSH!Whale was furious. “You are a meddling, incompetent BUFFOON!” he roared. “You have RUINED my act. Now GET OUT OF MY SIGHT!” Whale was pretty scary when he was angry. Walrus turned tail and fled. He hid behind a snowdrift, feeling sad and very sorry.4The show will go on! The show was over. Fox and Polar Bear and Seal and Whale stood in a line, blushing and wishing that Walrus had not ruined everything. But everyone was cheering like mad. “Well done, Polar Bear, well done, Seal! Well done, Fox and Whale!” they shouted. “But where"s the clown? Why isn"t he here? Where"s Walrus?” Behind is snowdrift, Walrus heard the cheers. Were they really cheering for him, too? Yes! They were! He shuffled up to Fox and Polar Bear and Seal And Whale.“I"m very sorry,” he said.“So you should be,” said Fox.“At least everyone thought you were part of the show,” said Polar Bear.“I suppose,” said Seal, “if Walrus was really part of our next show, it couldn"t be any worse.”Walrus was overjoyed. “Me?” he cried. “Truly? Can I be part of the show? Can I join in next time?”“Yes,” said Whale. “It will be a lot safer that way. You can be the clown. As long as you do it properly.”Walrus practiced hard and became a very good clown indeed. Now he is so good that Fox, Polar Bear, Seal and Whale are glad he is taking part.Sometimes, when Walrus is clowning around, they really just can"t stop themselves. They have to join in too!希望能帮到你，谢谢！

Walrus Joins In 1 What will Walrus do? Everyone at the North Pole was very excited. There was going to be a show and ANYONE could be in it. ‘I will do skating," said Arctic Fox. ‘I"m good at that!" ‘I"ll do tumbling," said Polar Bear. ‘No one tumbles quite like me!" ‘I"ll do singing," said Seal. ‘Everyone says I have a very fine voice!" ‘Then I"ll do diving," said Whale. ‘I won a prize for diving at school, you know!" They all looked at Walrus. ‘What will YOU do?" they asked. But Walrus was not good at anything. He wasn"t good at skating, and he wasn"t good at tumbling. He was terrible at singing, and when he tried to dive, he always got water up his nose. He sat and chewed his whiskers sadly. ‘Never mind," said Arctic Fox. ‘You can watch us." Arctic Fox and Polar Bear and Seal and Whale practised hard for the big show. Walrus hid behind a snowdrift and watched, and chewed his whiskers. He wished he was good at something. 2 The big night At last, the big night arrived. Everyone sat down and waited for the show to begin. Walrus sat in the front row. He was very excited. Fox came onto the ice and bowed. Everyone cheered. Then Fox began to skate. Fox skated forwards and backwards and sideways. She skated in perfect circles and figures of eight. She was elegant and amazing! Walrus watched and he loved what he saw. Fox made it all look so easy. Walrus was sure that if he really tried he could skate just like Fox. He couldn"t stop himself. He just had to leap onto the ice and join in with Fox. “I can skate,” he cried. “Look at me!” But Walrus couldn"t skate at all. He could only trip up and fall over. He bumped into Fox, and Fox went flat on her face. FLOMP! Fox was very upset. “Walrus has RUINED my act,” she wailed. Next, it was Polar Bear"s turn. He rolled out across the ice like a big, white snowball. Everyone clapped wildly. Then Polar Bear began to tumble. He did jumps and spins and somersaults, and stood on his head. Walrus watched and he loved what he saw. Polar Bear made it all look such fun. Walrus was sure that this time, if he really tried, he could tumble just like Polar Bear. All of a sudden, Walrus just couldn"t stop himself, and he leaped onto the ice. “I can tumble too,” he cried. “Look at me!” But Walrus couldn"t tumble at all. He could only trip up and fall over. He tripped up Polar Bear, who came down with a WALLOP! Of course, Polar Bear was pretty angry. “Walrus has RUINED my act,” he wailed. 3 From bad to worse It was Seal"s turn next. She gave Walrus a don"t-you-dare stare, and then she started to sing: “O, how many heart rejoices when I see the Northern Lights. My ear is filled with voices sweetly singing in the night!” Walrus listened. What a beautiful song! Surely if he really tried, he could sing as beautifully as Seal? Oh, dear. Walrus just couldn"t stop himself again. “I know that song,” he cried. “I can sing it too!” He leaped up and started singing along with Seal. But Walrus couldn"t sing! He sounded terrible. In fact, he sounded like a rusty old bucket. Seal stopped singing and burst into floods of tears. “Walrus has RUINED my song,” she wailed. Wale was last. He was pretty certain that Walrus could not ruin his act. Whale leaped high out of the water. Then he fell back with an enormous splash! Walrus watched. He wished that he could dive like that. His flippers began to twitch, and his whiskers bristled with excitement. He tried and tried his very best not to join in. But then he had a brilliant idea. “I"ll hold my nose when I dive,” he thought. “Then the water won"t go up it!” Walrus just couldn"t stop himself. He had to join in. “Everybody, look at me!” he cried, as he leaped into the water. “I can dive too!” But just at that moment, Whale was getting ready to spout a big jet of water. WHOOSH! Whale spouted Walrus high into the air! Everyone was watching Walrus now. They clapped and cheered as Walrus landed back in the sea with a SPLOSH! Whale was furious. “You are a meddling, incompetent BUFFOON!” he roared. “You have RUINED my act. Now GET OUT OF MY SIGHT!” Whale was pretty scary when he was angry. Walrus turned tail and fled. He hid behind a snowdrift, feeling sad and very sorry. 4 The show will go on! The show was over. Fox and Polar Bear and Seal and Whale stood in a line, blushing and wishing that Walrus had not ruined everything. But everyone was cheering like mad. “Well done, Polar Bear, well done, Seal! Well done, Fox and Whale!” they shouted. “But where"s the clown? Why isn"t he here? Where"s Walrus?” Behind is snowdrift, Walrus heard the cheers. Were they really cheering for him, too? Yes! They were! He shuffled up to Fox and Polar Bear and Seal And Whale. “I"m very sorry,” he said. “So you should be,” said Fox. “At least everyone thought you were part of the show,” said Polar Bear. “I suppose,” said Seal, “if Walrus was really part of our next show, it couldn"t be any worse.” Walrus was overjoyed. “Me?” he cried. “Truly? Can I be part of the show? Can I join in next time?” “Yes,” said Whale. “It will be a lot safer that way. You can be the clown. As long as you do it properly.” Walrus practiced hard and became a very good clown indeed. Now he is so good that Fox, Polar Bear, Seal and Whale are glad he is taking part. Sometimes, when Walrus is clowning around, they really just can"t stop themselves. They have to join in too! 希望能帮到你,谢谢!

对了额，。。，

考得是too..to.. 太...而不能... 暗示不能够加入。 如果是D选项，有军队招收小孩当兵的嫌疑啊

1.sign up for sign up for意思报名参加，签约参加工作, 签约雇用 2．take part intake part in 意为“参加，参与（某事物或某活动，如movement, revolution, meeting, conference, conversation, war等）”。例如：Switzerland didn"t take part in this war. 瑞士没有参加这次战争。How many countries will take part in the World Cup? 有多少个国家要参加世界杯？3．join injoin in sth. / doing sth.意为“参加”，后面接的宾语一般是表示竞赛、娱乐、游戏等活动的名词或V-ing。例如：Can I join in the game? 我可以参加这个游戏吗？They all join in singing the song. 他们一起唱这首歌。

sign up注册sign in登陆join now现在加入rgister now现在登记望采纳

第一个单词应该是join吧？ connect是指“认为有联系或有关系”，此时常用于被动语态，或指使用工具把一物与另一物“连结”起来，常与to连用。如：I always considered your brother to be connected with the crime.我总是认为你的哥哥和犯罪有关。join在表示“连接”时与link为同义词，指直接连接（两个以上的东西）；但在用法上不同，常见的用法是join ... to ...指“把……与……连接起来”。如：The island is joined to the mainland by a bridge.岛上有座桥与大陆相连。combine指为某一目的而把两个或两个以上的东西混合在一起。如：They combined the two companies.他们合并了两家公司。

如： I always considered your brother to be connected with the crime. 我总是认为你的哥哥和犯罪有关。 join在表示“连接”时与link为同义词，指直接连接（两个以上的东西）；但在用法上不同，常见的用法是join ... to ...指“把……与……连接起来”。如： The island is joined to the mainland by a bridge. 岛上有座桥与大陆相连。 combine指为某一目的而把两个或两个以上的东西混合在一起。如： They combined the two companies. 他们合并了两家公司。 [答] 此句翻译为：我们必须建设自己的生活，为自己思考，为自己说话。speak for oneself表示“自己表达自己的意思”。如： 另外，speak for itself / themselves意为“无需说明；不言自喻”。如： The events of that evening speak for themselves. 那天晚上发生的事不用说也很清楚。 speak for yourself表示“你只能代表你自己；你不能代表别人”。如： —We all played very badly. 我们在场上表现得都很差劲。

如： I always considered your brother to be connected with the crime. 我总是认为你的哥哥和犯罪有关。 join在表示“连接”时与link为同义词，指直接连接（两个以上的东西）；但在用法上不同，常见的用法是join ... to ...指“把……与……连接起来”。如： The island is joined to the mainland by a bridge. 岛上有座桥与大陆相连。 combine指为某一目的而把两个或两个以上的东西混合在一起。如： They combined the two companies. 他们合并了两家公司。 [答] 此句翻译为：我们必须建设自己的生活，为自己思考，为自己说话。speak for oneself表示“自己表达自己的意思”。如： 另外，speak for itself / themselves意为“无需说明；不言自喻”。如： The events of that evening speak for themselves. 那天晚上发生的事不用说也很清楚。 speak for yourself表示“你只能代表你自己；你不能代表别人”。如： —We all played very badly. 我们在场上表现得都很差劲。

如： I always considered your brother to be connected with the crime. 我总是认为你的哥哥和犯罪有关。 join在表示“连接”时与link为同义词，指直接连接（两个以上的东西）；但在用法上不同，常见的用法是join ... to ...指“把……与……连接起来”。如： The island is joined to the mainland by a bridge. 岛上有座桥与大陆相连。 combine指为某一目的而把两个或两个以上的东西混合在一起。如： They combined the two companies. 他们合并了两家公司。 [答] 此句翻译为：我们必须建设自己的生活，为自己思考，为自己说话。speak for oneself表示“自己表达自己的意思”。如： 另外，speak for itself / themselves意为“无需说明；不言自喻”。如： The events of that evening speak for themselves. 那天晚上发生的事不用说也很清楚。 speak for yourself表示“你只能代表你自己；你不能代表别人”。如： —We all played very badly. 我们在场上表现得都很差劲。

joined

java中string类concat方法和+的区别如下：concat：将指定字符串连接到此字符串的结尾。如果参数字符串的长度为 0，则返回此 String 对象。否则，创建一个新的 String 对象，用来表示由此 String，对象表示的字符序列和参数字符串表示的字符序列连接而成的字符序列。示例： 123"cares".concat("s") returns "caress""to".concat("get").concat("her") returns "together"参数：str - 连接到此 String 结尾的 String。返回：一个字符串，它表示在此对象字符后连接字符串参数字符而成的字符。+可以把任何类型的数据连接起来

今天就来讲三个文本连接函数：CONCATENATE、CONCAT、TEXTJOIN。这三个函数可以实现添加固定文本、带格式内容、区域合并、固定间隔符合并等各种连接。CONCATENATE函数一、函数简介功能：将两个或多个文本字符串联接为一个字符串。语法：CONCATENATE(text1, [text2], ...) text1（必需）：要联接的第一个项目。项目可以是文本值、数字或单元格引用； Text2, ... （可选）：要联接的其他文本项目。最多可以有 255 个项目，总共最多支持 8,192 个字符。二、用法举例CONCATENATE和文本连接符的用法基本一样，所以不再展开讲述，只把结果写到此处，详细介绍可参考昨天的文章。1、基本用法：直接合并多个单元格内容，使用公式：=CONCATENATE(A1,B1,C1,D1,E1,F1)。如下图：如果需要连接进固定文本内容，使用公式：=CONCATENATE("敬请关注：",A3,B3,C3,D3,E3,F3)。如下图：2、合并后换行：公式：=CONCATENATE("敬请关注：",CHAR(10),A1,B1,C1)结果如下图：3、合并带格式的内容：公式：=CONCATENATE(A2,TEXT(B2,"e年m月"),$C$1,TEXT(C2,"#,##0.00"),$D$1,TEXT(D2,"0.0%"))实现“日期”、“千分位分隔符”“百分比”等格式文本的链接。结果如下图：4、合并列实现多条件查找：在G2单元格输入公式：=SUMPRODUCT((CONCATENATE(A2:A13,B2:B13)=CONCATENATE(E2,F2))*C2:C13)即可完成查找。但，在实际多条件查询应用中，一般选择用文本连接符&。&与CONCATENATE函数的弊端：不管是文本连接符，还是CONCATENATE函数，在连接数据时，参数都必须是单元格，而不能是区域，所以，如果连接内容较多时，公式会非常长，写起来很麻烦！比如：=A2&B2&C2&D2&E2&F2&G2&H2&I2&J2；=CONCATENATE(A2,B2,C2,D2,E2,F2,G2,H2,I2,J2)因此，新版EXCEL2016出现了可以合并区域的文本连接函数：CONCAT和TEXTJOIN以上两个麻烦的公式可以简化为：=CONCAT(A1:J1)或=TEXTJOIN("",,A1:J1)，是不是简单了许多？CONCAT函数一、函数简介功能：将区域文本（或单个文本）联接为一个字符串。语法：CONCAT （文本 1，[文本 2]，...) 文本 1 （必需）：要合并的文本项。字符串或字符串数组，如单元格的区域中。 [文本 2，...] （可选）：要联接的额外的文本项。可以有 253 文本项的文本参数的最大值。二、用法举例1、合并区域：同行合并公式：=CONCAT(A1:L1)。结果如下图：行列区域合并公式：=CONCAT(A1:L4).结果如下图：2、加分隔符合并区域：公式：=CONCAT(A1:F1&" ")，（此处加空白分隔符）结果如下图：特别注意：此公式是数组公式，需要按<Ctrl+Shift+Enter>组合键结束。3、条件筛选区域合并：CONCAT函数还可以用于符合某些条件的数据合并，不如以下动图，筛选的是各个部门考核优秀的员工名单，而且，如果源数据有变化，结果也会跟着更新：公式：{=CONCAT(IF((A2:A16=E2)*(C2:C16="优秀"),B2:B16&"，",""))}，（按<Ctrl+Shift+Enter>组合键结束）该公式的含义是：凡是满足条件：部门列中等于E2部门、考核结果列中等于优秀的就合并。TEXTJOIN函数CONCAT函数能完成的功能，TEXTJOIN 函数都能完成，并且TEXTJOIN 函数在忽略空白单元格、合理应用分隔符方面更胜一筹。一、函数简介功能：将从多个区域和/或字符串文本结合在一起，包括您指定将结合每个文本值之间的分隔符。语法：TEXTJOIN （分隔符， 是否忽略空白单元格，文本 1，[文本 2]，...） 分隔符（必需）:文本字符串，或者为空，或用双引号引起来的一个或多个字符，或对有效文本字符串的引用。如果提供一个数字，则将被视为文本。 是否忽略空白单元格（必需）:如果为 TRUE（或1），则忽略空白单元格； 文本1（必需）：要联接的文本项。文本字符串或字符串数组，如单元格区域中。 [文本2, ...]（可选）：要联接的其他文本项。文本项最多可以包含252 个文本参数text1。每个参数可以是一个文本字符串或字符串数组，如单元格区域。二、用法举例1、合并区域：同行合并公式：=TEXTJOIN("、",1,A1:F1)。含义是：忽略空白单元格合并，并用“、u2019”分隔。结果如下图：行列区域合并公式：=TEXTJOIN("",1,A1:L4)。结果如下图：2、条件筛选区域合并：TEXTJOIN函数还可以用于合并满足某一条件的单元格合并，比如下面动图，展示的是同一部门员工的合并，而且，随着员工部门调动，结果也随之改变：公式：{=TEXTJOIN("、",1,IF($A$2:$A$16=D2,$B$2:$B$16,""))}，（按<Ctrl+Shift+Enter>组合键结束）该公式的含义是：合并部门列同一部门的员工。备注CONCAT和TEXTJOIN函数是EXCEL2016特有的，而且，并不是每一个2016版本都有。

毛问了，你街毛线啊！！！

1.attend是正式用语，一般指参加会议、典礼、婚礼、葬礼及去听课、听报告等等。例句：　①MrSmithattendedthemeeting.史密斯先生出席了这次会议。　②Didheattendthesocialgetheringyesterday?昨天他出席社交聚会了吗？　③Ihopeyouattendalawschool.我希望你上法律学校。2.join指加入某党派、某组织或某社会团体，以及参军等，还可表示参与某种活动。例句：①IwillneverforgetthedaywhenIjoinedtheParty.我永远也忘不了入党的那一天。②Willyoujoinusfordinner?请你和我们一起吃饭好吗？3.joinin通常指参加某种活动，尤其指和其他人一起参加某项活动。例句：①MayIjoininthegame?我可以参加这个比赛吗？②Ihopeyou”llalljoininthediscussion.我希望你们大家都参加讨论。如果说"与某人一起做某事"、"和……在一起"则用joinsb.insth./doingsth.。例句：①Wouldyoujoinmeinawalk?和我一起去散步好吗？②Willyoujoinusinagameofcards?你愿意和我们一起玩牌吗？③I”lljoinyouinafewminutes.我过几分钟将和你在一起。4.takepartin指参加群众性活动、会议、劳动、游行等，往往指参加者持有积极的态度，起一份作用，有时与joinin可互换。例句：①WillyoutakepartintheEnglishevening?同我们一起参加英语晚会好吗？②Allthestudentstookanactivepartinthethoroughcleaning.所有的学生都积极参加了大扫除。③Weshouldtakeanactivepartinschoolactivities.我们应该积极参加学校的活动。

takepartin的主语是活动的主要参加者（主体）：Threeteamstookpark(inthegame).不谈具体参加内容时，不用in短语。join和joinin的意思是，在别人进行活动时的加入者（客体）：Weareplayingbasketball.Comeandjoinus.join的宾语一般是人、团体、组织等；joinin的宾语一般是活动。

所谓的semi-join是指semi-join子查询。 当一张表在另一张表找到匹配的记录之后，半连接（semi-jion）返回第一张表中的记录。 与条件连接相反，即使在右节点中找到几条匹配的记录，左节点 的表也只会返回一条记录。另外，右节点的表一条记录也不会返回。半连接通常使用IN 或 EXISTS 作为连接条件。 该子查询具有如下结构： SELECT ... FROM outer_tables WHERE expr IN (SELECT ... FROM inner_tables ...) AND ... 这种查询的特点是我们只关心outer_table中与semi-join相匹配的记录。 换句话说，最后的结果集是在outer_tables中的，而semi-join的作用只是对outer_tables中的记录进行筛选。这也是我们进行 semi-join优化的基础，即我们只需要从semi-join中获取到最少量的足以对outer_tables记录进行筛选的信息就足够了。 所谓的最少量，体现到优化策略上就是如何去重。 以如下语句为例： 可能返回的结果集如下： China(Beijin), China(Shanghai), France(Paris)... 我们可以看到这里有2个China，分别来至2条城市记录Beijin和Shanghai, 但实际上我们只需要1个China就足够对outer_table Country进行筛选了。所以我们需要去重。 Mysql支持的semi-join策略主要有5个，它们分别为： 直接转为join Convert the subquery to a join, or use table pullout and run the query as an inner join between subquery tables and outer tables. Table pullout pulls a table out from the subquery to the outer query. 只选用内部表的第1条与外表匹配的记录。 FirstMatch: When scanning the inner tables for row combinations and there are multiple instances of a given value group, choose one rather than returning them all. This "shortcuts" scanning and eliminates production of unnecessary rows. 把inner-table数据基于索引进行分组，取每组第一条数据进行匹配。 LooseScan: Scan a subquery table using an index that enables a single value to be chosen from each subquery"s value group. 使用临时表对semi-join产生的结果集去重。 Duplicate Weedout: Run the semijoin as if it was a join and remove duplicate records using a temporary table. Using index; Start temporary Using where Using index; End temporary Start temporary, End temporary 表示半连接中使用了DuplicateWeedout策略的临时表 将inner-table去重固化成临时表，遍历固化表，然后在outer-table上寻找匹配。 Materialize the subquery into an indexed temporary table that is used to perform a join, where the index is used to remove duplicates. The index might also be used later for lookups when joining the temporary table with the outer tables; if not, the table is scanned. For more information about materialization, see Section 8.2.2.2, “Optimizing Subqueries with Materialization”. mysql> SELECT @@optimizer_switchG *************************** 1. row *************************** @@optimizer_switch: index_merge=on,index_merge_union=on, index_merge_sort_union=on, index_merge_intersection=on, engine_condition_pushdown=on, index_condition_pushdown=on, mrr=on,mrr_cost_based=on, block_nested_loop=on,batched_key_access=off, materialization=on,semijoin=on,loosescan=on, firstmatch=on, subquery_materialization_cost_based=on, use_index_extensions=on 下列join查询可能出现重复 However, the result lists each class once for each enrolled student. For the question being asked, this is unnecessary duplication of information. 但是，结果会为每个注册的学生列出一次每个班级。对于正在问的问题，这是不必要的信息重复。 Assuming that class_num is a primary key in the class table, duplicate suppression is possible by using SELECT DISTINCT, but it is inefficient to generate all matching rows first only to eliminate duplicates later. The same duplicate-free result can be obtained by using a subquery: 假设class_num是class表中的主键，通过使用SELECT DISTINCT可以抑制重复，但是先生成所有匹配的行，然后再消除重复是低效的。 使用子查询可以获得相同的无重复结果: 在这里，优化器可以认识到IN子句要求子查询只从花名册表中返回每个类号的一个实例。在这种情况下，查询可以使用半连接；也就是说，只返回类中与花名册中的行匹配的每一行的一个实例的操作。 包含EXISTS子查询谓词的以下语句相当于包含IN子查询谓词的前一条语句: 有些join会出现重复，我们第一时间想到的是使用DISTINCT去重。但是这里是先join后去重，效率低。 我们完全可以先去重后join。 so，使用in或EXISTS子查询的Semijoin的方式可以优化之 1、使用join distinct 2、使用Semijoin ，d表出现 FirstMatch(a)

join party

前面是入党后面是加入派对

join in the pary或者take in the party 参加活动用join in 或take part in 加入组织 用join 比如join the army