1.显示不重复的的值：
SELECT DISTINCT 列名 FROM 表名
2.显示前几条数据
SQLSERVER：SELECT TOP number|percent column_name|* FROM table_name
例：
SELECT TOP 5 * FROM table_name;
SELECT TOP 50 PERCENT column_name FROM table_name;
MYSQL: SELECT * FROM table_name LIMIT number
例：查询前十名的 SELECT * FROM table_name LIMIT 10;
ORACLE: SELECT * FROM table_name WHERE ROWNUM <=10
3.like+通配符
SELECT * FROM table_name where column_name LIKE pattern
% 替代一个或多个字符
_ 替代一个字符
[charlist] 字符列中的任何单一字符
[^charlist]或者[!charlist] 不在字符列中的任何单一字符
4.IN
SELECT * FROM table_name WHERE column_name IN ('value1', 'value2');
5.BETWEEN
SELECT * FROM table_name WHERE column_name BETWEEN VALUE1 AND VALUE2
6.别名Alias
表别名：SELECT * FROM table_name AS alias_name WHERE alias_name = abc
列别名：SELECT column1_name AS alias_name1, column2_name AS alias_name2 FROM table_name;
7.JOIN
JOIN: 如果表中有至少一个匹配，则返回行
LEFT JOIN: 即使右表中没有匹配，也从左表返回所有的行
RIGHT JOIN: 即使左表中没有匹配，也从右表返回所有的行
FULL JOIN: 只要其中一个表中存在匹配，就返回行
内连接JOIN = INTER JOIN
SELECT column_name(s) FROM tableA INTER JOIN tableB ON tableA.column_name= tableB.column_name;
左连接 LEFT JOIN = LEFT OUTER JOIN
SELECT column_name(s) FROM tableA LEFT JOIN tableB ON tableA.column_name = tableB.column_name;
右连接 RIGHT JOIN = RIGHT OUTER JOIN
SELECT column_name(s) FROM tableA RIGHT JOIN tableA ON tableA.column_name = tableB.column_name;
FULL JOIN = FULL OUTER JOIN
SELECT column_name(s) FROM tableA FULL JOIN tableB ON tableA.column_name = tableB.column_name;
8.UNION和UNION ALL
UNION 操作符用于合并两个或多个 SELECT 语句的结果集。
请注意，UNION 内部的 SELECT 语句必须拥有相同数量的列。列也必须拥有相似的数据类型。同时，每条 SELECT 语句中的列的顺序必须相同。
默认地，UNION 操作符选取不同的值。如果允许重复的值，请使用 UNION ALL。
SELECT column_names FROM tableA UNION SELECT column_names FROM tableB
SELECT column_names FROM tableA UNION ALL SELECT column_names FROM tableB
9.SELECT INTO
SELECT INTO 语句从一个表中选取数据，然后把数据插入另一个表中。
SELECT INTO 语句常用于创建表的备份复件或者用于对记录进行存档。
SELECT * INTO new_table IN [externaldatabase] FROM old_table;
SELECT column_name(s) INTO new_table IN [externaldatabase] FROM old_table;
10.CREATE DATABASE
CREATE DATABASE database_name;
11.CREATE TABLE
CREATE TABLE table_name
{
列名称1，数据类型；
列名称n，数据类型；
}
