单表查询：
SELECT
[ DISTINCT ]
[SQL_CACHE | SQL_NO_CACHE]
select_expr [,select_expr ...]
[FROM table_references [ WHERE where_condition ]
[GROUP BY {col_name | expr |}] [HAVING where_condition]
[ORDER BY [ASC | DESC]]
DISTINCT：数据去重
SQL_CACHE：指定存入缓存
SQL_NO_CACHE：指定进行缓存
WHERE子句：指定条件，实现过滤功能
过滤条件：
算数运算：+ - * / % > < >= <=
逻辑运算：AND：与运算 OR：或运算 NOT：非运算
字符比较：= !=
列表元素比较：IN (元素1,元素2,元素3,...)
IS NULL：空
IS NOT NULL：非空
LIKE：模糊匹配，能不用尽量不用
RLIKE：基于正则的模糊匹配，能不用尽量不用
GROUP BY子句：更具指定的条件把匹配的结果进行分组，实现"聚合"运算
常用函数：
sum(条件)
max(条件)
min(条件)
avg(条件)
count(条件)
HAVING：对GROUP BY之后的结果进行过滤
示例：
mysql> CREATE DATABASE mydb;
mysql> USE mydb;
mysql> CREATE TABLE test (id int(50),name varchar(50),qq int(15),gender char(2));
mysql> INSERT INTO test VALUES(1,'zhangshan',12345,'F');
mysql> INSERT INTO test VALUES(2,'lisi',123142,'F');
mysql> INSERT INTO test VALUES(3,'zsf',124312,'M');
mysql> INSERT INTO test VALUES(4,'yyy',124312,'F');
mysql> INSERT INTO test VALUES(5,'ytt',124312,'M');
单表查询示例：
mysql> SELECT * FROM test WHERE id>2 AND id<5;
mysql> SELECT * FROM test WHERE id>2 GROUP BY gender;
mysql> SELECT sum(id),gender FROM test GROUP BY gender;
mysql> SELECT * FROM test GROUP BY gender HAVING id>2;
多表关联查询：
mysql> CREATE TABLE test2 (emain varchar(200),age int(10));
mysql> INSERT INTO test2 values('123@qq.com',10);
mysql> INSERT INTO test2 values('456@qq.com',15);
mysql> INSERT INTO test2 values('789@qq.com',20);
mysql> SELECT test.id,test2.age FROM test,test2 WHERE test.id>2 AND test2.age>10;
test.id
test2.age
test
test2
test.id>2 AND test2.age
mysql> SELECT * FROM test,test2 WHERE test.id>2 AND test2.age>10;
