表操作

1、创建表（DDL）

• CREATE [TEMPORARY] TABLE [IF NOT EXISTS] tbl_name
  (create_definition,...)
  [table_options]
  [partition_options]

    CREATE TABLE mytest(id INT UNSIGNED NOT NULL PRIMARY KEY,name VARCHAR(30) NOT NULL,age TINYINT UNSIGNED NOT NULL,gender ENUM('F','M'));
  

• CREATE [TEMPORARY] TABLE [IF NOT EXISTS] tbl_name
  [(create_definition,...)]
  [table_options]
  [partition_options]
  select_statement

  CREATE TABLE mytest1 SELECT * FROM mytest；
  

• CREATE [TEMPORARY] TABLE [IF NOT EXISTS] tbl_name
  { LIKE old_tbl_name | (LIKE old_tbl_name) }

   CREATE TABLE mytest2 LIKE mytest;
  

2、插入数据(DML)

• INSERT [LOW_PRIORITY | DELAYED | HIGH_PRIORITY] [IGNORE]
  [INTO] tbl_name [(col_name,...)]
  {VALUES | VALUE} ({expr | DEFAULT},...),(...),...
  [ ON DUPLICATE KEY UPDATE
  col_name=expr
  [, col_name=expr] ... ]

   INSERT students(Name,Age,Gender,ClassID,TeacherID)  VALUE("Jrc",22,'M',NULL,NULL);
  

• INSERT [LOW_PRIORITY | DELAYED | HIGH_PRIORITY] [IGNORE]
  [INTO] tbl_name
  SET col_name={expr | DEFAULT}, ...
  [ ON DUPLICATE KEY UPDATE
  col_name=expr
  [, col_name=expr] ... ]

   INSERT students SET Name="Zl",Age=23,Gender="F",ClassID=NULL,TeacherID=NULL;
  

• INSERT [LOW_PRIORITY | HIGH_PRIORITY] [IGNORE]
  [INTO] tbl_name [(col_name,...)]
  SELECT ...
  [ ON DUPLICATE KEY UPDATE
  col_name=expr
  [, col_name=expr] ... ]

3、删除数据（DML）

• Single-table syntax:
  DELETE [LOW_PRIORITY] [QUICK] [IGNORE] FROM tbl_name
  [WHERE where_condition]
  [ORDER BY ...]
  [LIMIT row_count]

  DELETE FROM students WHERE Name='Jrc';
  

• Multiple-table syntax:
  DELETE [LOW_PRIORITY] [QUICK] [IGNORE]
  tbl_name[.] [, tbl_name[.]] ...
  FROM table_references
  [WHERE where_condition]

  DELETE [LOW_PRIORITY] [QUICK] [IGNORE]
  tbl_name[.] [, tbl_name[.]] ...
  FROM table_references
  [WHERE where_condition]

4、更新数据（DML）

• Single-table syntax:
  UPDATE [LOW_PRIORITY] [IGNORE] table_reference
  SET col_name1={expr1|DEFAULT} [, col_name2={expr2|DEFAULT}] ...
  [WHERE where_condition]
  [ORDER BY ...]
  [LIMIT row_count]

  UPDATE students set Name='Jrc',Age=22,Gender='M' WHERE Name='Zl';
  

• Multiple-table syntax:
  UPDATE [LOW_PRIORITY] [IGNORE] table_references
  SET col_name1={expr1|DEFAULT} [, col_name2={expr2|DEFAULT}] ...
  [WHERE where_condition]

5、查询数据（DML）

• 简单查询
  SELECT
  [ALL | DISTINCT | DISTINCTROW ]
  [HIGH_PRIORITY]
  [STRAIGHT_JOIN]
  [SQL_SMALL_RESULT] [SQL_BIG_RESULT] [SQL_BUFFER_RESULT]
  [SQL_CACHE | SQL_NO_CACHE] [SQL_CALC_FOUND_ROWS]
  select_expr [, select_expr ...]
  [FROM table_references
  [WHERE where_condition]
  [GROUP BY {col_name | expr | position}
  [ASC | DESC], ... [WITH ROLLUP]]
  [HAVING where_condition]
  [ORDER BY {col_name | expr | position}
  [ASC | DESC], ...]
  [LIMIT {[offset,] row_count | row_count OFFSET offset}]

SELECT语句的执行流程：

    FROM --> WHERE --> GROUP BY --> HAVING --> ORDER BY --> SELECT --LIMIT

DISTINCT：数据去重；
SQL_CACHE：指定存储查询结果与缓存中；
SQL_NO_CACHE：查询结果不予缓存；
 query_cache_type值为ON时，查询缓存功能打开，SELECT的结果符合缓存即会缓存；
 query_cache_tyoe值为DEMAND时，指定SQL_CACHE的SELECT语句才会缓存；

字段显示可以显示别名：
  col1 AS alias1,col2 AS alias2,......

• WHERE子句：指明过滤条件以实现”选择“的功能；
    过滤条件：布尔型表达式;
    算术操作符：+，-，*，、，%
    比较操作符：=，！=，< , >
      BETWEEN min_num AND max_num
      IN (element1,element2,...)
      IS NULL
      IS NOT NULL
      LIKE：
        %：任意长度的任意字符；
        _：任意单个字符；
      RLIKE
      REGEXP
    逻辑操作符：
      NOT
      AND
      OR
      XOR

• GROUP BY：根据指定的条件把查询结果进行“分组”以用于做“聚合”运算；
    avg(),max(),min(),count(),sum()
    HAVING：对分组聚合运算后的结果做指定条件过滤；

   SELECT ClassID,count(StuID) AS NOS FROM students GROUP BY ClassID HAVING NOS>=3;
  

• ORDER BY：根据指定的字段对查询结果进行排序；
    升序：ASC
    降序：DESC

• LIMIT [[offset,]row_count]]：对查询的结果进行输出行数限制；

    SELECT * FROM students LIMIT 10,3;
  

对查询结果中的数据请求施加锁：
  FOR UPDATE：写锁，排他锁；
  LOCK IN SHARE MODE：读锁，共享锁；

• 多表查询
    交叉连接：笛卡尔乘积
    内连接：
       等值连接：让表之间的字段以“等值”建立连接关系；
      不等值连接
      自然连接
      自连接
    外连接：
      左外连接：
        FROM tb1 LEFT JOIN tb2 ON tb1.col1=tb2.col
        FROM tb1 RIGHT JOIN tb2 ON tb1.col1=tb2.col

  SELECT students.Name,classes.Class FROM classes,students WHERE classes.ClassID=students.ClassID;
  

• 子查询：在查询语句嵌套着查询语句
    基于某语句的查询结果再次进行查询

  用在WHERE子句中的子查询：
    （1）用于比较表达式中的子查询，子查询仅能返回单个值

    SELECT Name,Age FROM students WHERE Age>(SELECT avg(Age) FROM students);

    （2）用于IN中的子查询，子查询应该单键查询并返回一个或多个值从而构成列表；

    SELECT Name,Age FROM students WHERE Age IN (SELECT Age FROM teachers);

    （3）用于EXISTS

  用于FROM子句中的子查询

• 联合查询：UNION

   SELECT Name,Age FROM students UNION SELECT Name,Age FROM teachers;