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Lesson One A private conversationLast week, I went to the theatre. I had a very good seat.The play was very interesting. I didn’t enjoy it.A young man and a young woman were sitting behind me.They were talking loudly. I got very angry.I couldn’t hear the actors. I turned round. I look at theMan and 阅读全文
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Lesson One hundred and forty one Sally’s first train rideLast week, my four-year-old daughter, Sally,Was invited to a children’s party.I decided to take her by train.Sally was very excitedBecause she had never travelled on a train before.She sat near the window and asked questionsAbout everything sh 阅读全文
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5.含有子查询的DML语句在数据操作语言中可以使用子查询语句，完成操作过程较复杂，功能较强的DML操作。（1）插入数据INSERT语句中使用子查询将某表的数据查询子集插入到另一个表中。语句格式：INSERT INTO <table_name> [(column_name [, column_name, ...])] subquery--subquery用于指定表提供数据的子查询--建立计算机专业学生students_computer表：CREATE TABLE students_computer(student_id NUMBER(5) CONSTRAINT student_com 阅读全文
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4.数据库完整性数据库完整性包括三个方面的约束规则，分别是实体完整性、参照完整性和自定义完整性约束规则。（1）实体完整性实体完整性是指关系的主属性，即表的主键不能为NULL，也不能取重复值。INSERT INTO students(name, specialty)VALUES ('王一','计算机');第1行出现错误：ORA-01400，无法将NULL插入（"SYSTEM"."STUDENTS"."STUDENT_ID"）。UPDATE students SET student_id = NULL--在 阅读全文
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数据操作语言DML包括INSERT、DELETE、UPDATE等语句。它们分别进行数据增、删、改的功能。Oracle处理DML语句的结果时，以事务（transaction）为单位进行，一个事务为一个工作的逻辑单元，是一个SQL语句序列，在执行每一个DML语句时，所有的操作都在内存中完成，在执行完一系列DML语句后，需要决定由DML语句所做的修改，是全部或部分地保留到硬盘文件上，还是全部撤销。一、数据操作语言1.插入数据INSERTINSERT语句的作用是将数据行追加到表或视图的基表中。INSERT INTO students --此情况可省略列名（顺序和数量一致）VALUES(10138,10 阅读全文