UPDATE

 1 --使用表达式更新一个单列的值
 2 UPDATE employee
 3     SET salary = salary*1.1
 4 WHERE department_id = 90
 5 
 6 --使用子查询更新一个单列的值
 7 UPDATE employee
 8     SET salary = (
 9         SELECT employee2.salary
10         FROM employee2 WHERE employee_id = 10
11     )
12 WHERE employee_id = 90
13 
14 --通过WHERE子句中使用子查询确定要更新的数据行来更新单列的值
15 
16 
17 --通过使用SELECT语句定义表及列的值来更新表
18 UPDATE (
19     SELECT col1,col2,...
20     FROM table_name1
21     WHERE ......
22 )
23 SET col_value = new_col_value
24 
25 --通过子查询更新多列
26 UPDATE
27     SET (col1,col2) = (
28         SELECT col2,col2 FROM table1
29     )
30 WHERE ...