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THE internet ENCYCLOPEDIA 1 volume 3 phần 5

Bằng cách loại trừ những chi tiết này, CP là một tài liệu ổn định và cao cấp. Nhiều CA có thể hoạt động theo một CP. Đây thường là trường hợp nhiều khi CA được duy trì bởi một doanh nghiệp, cùng hỗ trợ một cộng đồng duy nhất. Những người khác nhau sẽ sử dụng CP cho các lý do khác nhau. | DDL 359 that are stored in different tables such as salaried employee wages and hourly employee wages. It makes no sense to have number of tickets and ticket price joined via a union but SQL will probably allow it because they are both numeric columns. The syntax is select statement 1 UNION select statement 2 Minus The minus operation also directly ported from set theory removes matching rows from multiple result sets. This is useful in determine what is different between these results or what didn t sell while it was under a promotion. The syntax is select statement 1 MINUS select statement 2 DDL Create Table The create table statement allocates storage for data to be stored. This storage has a structure columns with datatypes and optional constraints defined. Once the table is created the table is ready to receive data via the INSERT statement see below . The syntax is straightforward create table TABLE_NAME column_element table_constraint For example to create the movie table located in Appendix A the following syntax was used CREATE TABLE MOVIE MOVIE_ID NUMBER NOT NULL MOVIE_TITLE VARCHAR2 50 NOT NULL INVENTORY_ID NUMBER NOT NULL MOVIE_RATING CHAR 5 NOT NULL PASS_ALLOWED CHAR 1 NOT NULL Keys When a database is modeled using entity-relationship techniques identifying attribute s are identified for all fundamental entities tables along with those attributes that relate two or more tables. The identifying attribute s uniquely distinguish one row of data from all others in the table. In SQL these identifying attribute s are identified to the database engine as a primary key Many database engines use this information to enforce the uniqueness property that is inherent in the definition. Attributes that tie relations together are known as foreign keys. Keys can be identified either when a table is created with the create table command or after via the alter table command . Create Index The create index statement allows the database engine to create an index .