8.笔记 MySQL学习——检索信息
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8.笔记 MySQL学习——检索信息
1. 相关表查找
查找表如下:
mysql> select * from student;
+--------+-----+------------+
| name | sex | student_id |
+--------+-----+------------+
| Kyle | M | 1 |
| Abby | F | 2 |
| Joseph | M | 3 |
+--------+-----+------------+
3 rows in set (0.00 sec)
mysql> select 2+2,'Hello,world' ,version();
+-----+-------------+-----------+
| 2+2 | Hello,world | version() |
+-----+-------------+-----------+
| 4| Hello,world | 5.7.10 |
+-----+-------------+-----------+
1 row in set (0.00 sec)
2. 指定检索条件
mysql> select * from score where score > 95;
+------------+----------+-------+
| student_id | event_id | score |
+------------+----------+-------+
| 5 | 3 | 97 |
| 18 | 3 | 96 |
| 1 | 6 | 100 |
| 5 | 6 | 97 |
| 11 | 6 | 98 |
| 16 | 6 | 98 |
+------------+----------+-------+
6 rows in set (0.00 sec)
mysql> select last_name,first_name,state frompresident where state='VA' or state='MA';
+------------+-------------+-------+
| last_name | first_name | state |
+------------+-------------+-------+
| Washington | George | VA |
| Adams | John | MA |
| Jefferson | Thomas | VA |
| Madison | James | VA |
| Monroe | James | VA |
| Adams | John Quincy | MA |
| Harrison | William H. | VA |
| Tyler | John | VA |
| Taylor | Zachary | VA |
| Wilson | Woodrow | VA |
| Kennedy | John F. | MA |
| Bush | George H.W. | MA |
+------------+-------------+-------+
12 rows in set (0.00 sec)
3. NULL值
mysql> select null<0,null=0,null<>0,null>0;
+---------+--------+---------+--------+
| null <0 | null=0 | null<>0 |null>0 |
+---------+--------+---------+--------+
| NULL | NULL | NULL | NULL |
+---------+--------+---------+--------+
1 row in set (0.00 sec)
NULL值是无值 或 未知值。
可以使用IS NULL 或IS NOT NULL 来查找NULL值。
4. 对查询结果排序
mysql> select last_name,first_name from presidentorder by last_name;
+------------+---------------+
| last_name | first_name |
+------------+---------------+
| Adams | John |
| Adams | John Quincy |
| Arthur | Chester A. |
| Buchanan | James |
| Bush | George W. |
| Bush | George H.W. |
| Carter | James E. |
| Cleveland | Grover |
| Clinton | William J. |
| Coolidge | Calvin |
| Eisenhower | Dwight D. |
| Fillmore | Millard |
| Ford | Gerald R. |
| Garfield | James A. |
| Grant | Ulysses S. |
| Harding | Warren G. |
| Harrison | William H. |
| Harrison | Benjamin |
| Hayes | Rutherford B. |
| Hoover | Herbert C. |
| Jackson | Andrew |
| Jefferson | Thomas |
| Johnson | Andrew |
| Johnson | Lyndon B. |
| Kennedy | John F. |
| Lincoln | Abraham |
| Madison | James |
| McKinley | William |
| Monroe | James |
| Nixon | Richard M. |
| Obama | Barack H. |
| Pierce | Franklin |
| Polk | James K. |
| Reagan | Ronald W. |
| Roosevelt | Franklin D. |
| Roosevelt | Theodore |
| Taft | William H. |
| Taylor | Zachary |
| Truman | Harry S |
| Tyler | John |
| Van Buren | Martin |
| Washington | George |
| Wilson | Woodrow |
+------------+---------------+
43 rows in set (0.00 sec)
MYSQL 的 SQL语句和 ORACLE的还是很像的。
5. 限制查询
mysql> select last_name,first_name,birth frompresident order by birth limit 5;
+------------+------------+------------+
| last_name | first_name | birth |
+------------+------------+------------+
| Washington | George | 1732-02-22 |
| Adams | John | 1735-10-30 |
| Jefferson | Thomas | 1743-04-13 |
| Madison | James | 1751-03-16 |
| Monroe | James | 1758-04-28 |
+------------+------------+------------+
5 rows in set (0.00 sec)
6. 对输出列进行计算和命名
mysql> select concat (first_name ,' ',last_name),concat (city,', ',state) from president;
+------------------------------------+--------------------------+
| concat (first_name ,' ',last_name) |concat (city,', ',state) |
+------------------------------------+--------------------------+
| George Washington | Wakefield, VA |
| John Adams | Braintree, MA |
| Thomas Jefferson | Albemarle County, VA |
| James Madison | Port Conway, VA |
| James Monroe | Westmoreland County,VA |
| John Quincy Adams | Braintree, MA |
| Andrew Jackson | Waxhaw settlement,SC |
| Martin Van Buren | Kinderhook, NY |
| William H. Harrison | Berkeley, VA |
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