3.29学习情况回顾
今天主要学习了MySQL数据库的单表主要查询方法。
一、MySQL数据库查询
- 单表查询的常用代码
(1)展示表结构:DESC d_book;
(2)展示表全部数据:SELECT * FROM d_book;
(3)展示表中部分字段数据:SELECT id,author FROM d_book;
(4)展示表中部分字段等于单个固定值数据:
(5)SELECT * FROM d_book WHERE publishing=‘北京十月文艺出版社’;
(6)SELECT * FROM d_product WHERE dang_price <=22;
(7)SELECT * FROM d_book WHERE total_page < 400;
(8)展示表中部分字段等于多个固定值数据:SELECT * FROM d_book WHERE author IN(‘莫言’,‘路遥’,‘南派三叔’,‘二月河’);
(9)位于某个值区间的数据(包含值):SELECT * FROM d_product WHERE dang_price BETWEEN 50 AND 100;
(10)模糊查询(百分号表示无限个字节的数据,下划线表示一个字节):
SELECT * FROM d_book WHERE author LIKE ‘张%’;
SELECT * FROM d_book WHERE author LIKE ‘__’;
(11)去重:SELECT DISTINCT publishing FROM d_book;
(12)排序—升序(不设置则默认升序):SELECT * FROM d_product ORDER BY dang_price ASC;
(13)排序—降序:SELECT * FROM d_product ORDER BY fixed_price DESC;
(14)通过字段分组:SELECT publishing,GROUP_CONCAT(author) FROM d_book GROUP BY publishing;
(15)将分组数据汇总到括号内字段:SELECT publishing,GROUP_CONCAT(author) FROM d_book GROUP BY publishing;
(16)limit数字几——展示前几行数据(注意:此处默认第一行是0,第二行是1……即limit5表示第0-4行):SELECT * FROM d_category WHERE turn=1 LIMIT 5;
(17)limit数字几1,数字几2——从第1几行开始,展示2几行数据(注意:此处默认第一行是0,第二行是1……即limit2,3表示从第2行开始,展示接下来3行数据):SELECT * FROM d_category WHERE turn=1 LIMIT 2,4;
(18)count()——计数(注意:括号内为时,计数结果包括null行,括号内为具体字段时,计数结果不包含null行):SELECT COUNT() FROM d_book;
求和、最大值、最小值、平均值:SELECT SUM(dang_price),MAX(dang_price),MIN(dang_price),AVG(dang_price) FROM d_product;
(19)having——对结果二次筛选:SELECT class,COUNT(class),AVG(height) FROM school GROUP BY class HAVING AVG(height)<160;
(20) ‘’ ——在字段后空格加引号,引号内中文,可显示字段名称为中文:SELECT empno ‘员工编号’ ,ename ‘员工姓名’ ,12sal ‘年薪’ FROM emp;
(21) ifnull(字段,0)——在对字段求和时,如果字段含有null值,则把null作为0计算:
SELECT empno ‘员工编号’ ,ename ‘员工姓名’ ,12*(sal+IFNULL(comm,0)) ‘年收入’ FROM emp;