文件名称：基本SELECT语句-SQL语言基础
文件大小：5.26MB
文件格式：PPT
更新时间：2024-05-15 13:24:45
SQL 基础
基本SELECT语句 * SELECT {*, column [alias],...} FROM table; SELECT 关键字标明取哪些列的数据. FROM 关键字标明从哪些表中取数据. 重要特征：表达式以分号结尾. * Select与from出现的最多，是计算机进行数据检索操作的比不和少的成分。必须同时在一起才能访问数据库。 表明命令行SQL查询程序已经完成。 DISTINCT剔除重复数据。ALL Basic SELECT Statement In its simplest form, a SELECT statement must include the following: A SELECT clause, which specifies the columns to be displayed A FROM clause, which specifies the table containing the columns listed in the SELECT clause In the syntax: SELECT is a list of one or more columns. DISTINCT suppresses duplicates. * selects all columns. column selects the named column. alias gives selected columns different headings. FROM table specifies the table containing the columns. Note: Throughout this course, the words keyword, clause, and statement are used. A keyword refers to an individual SQL element. For example, SELECT and FROM are keywords. A clause is a part of a SQL statement. For example, SELECT empno, ename, ... is a clause. A statement is a combination of two or more clauses. For example, SELECT * FROM emp is a SQL statement.