mysql 命令列操作（四）高階sql語句
阿新 • • 發佈：2018-12-09
1.union，連線兩個以上的select語句的結果組合到一個結果集中，多個select語句中相同的資料會被刪除，多個union的select語句必須列數相同，比如第一個select選擇兩個列，那麼後續所有union的select語句都必須是兩列，不要求列的屬性一致，可以用union all保留所有重複的資料。用於測試的兩個表資料如下：
mysql> select * from app; +----+------------+-------------------------+---------+ | id | app_name | url | country | +----+------------+-------------------------+---------+ | 1 | QQ APP | http://im.qq.com/ | CN | | 2 | 微博 APP | http://weibo.com/ | CN | | 3 | 淘寶 APP | https://www.taobao.com/ | CN | +----+------------+-------------------------+---------+ 3 rows in set (0.00 sec) mysql> select * from website -> ; +----+---------------+---------------------------+-------+---------+ | id | name | url | alexa | country | +----+---------------+---------------------------+-------+---------+ | 1 | Google | https://www.google.com/ | 1 | USA | | 2 | 淘寶 | https:/www.taobao.com/ | 13 | CN | | 3 | 菜鳥教程 | http://www.runoob.com | 4689 | CN | | 4 | 微博 | http://weibo.com/ | 20 | CN | | 5 | Facebook | https://www.facebook.com/ | 3 | USA | | 6 | stackoverflow | http://stackoverflow.com/ | 0 | IND | +----+---------------+---------------------------+-------+---------+ 6 rows in set (0.00 sec)
將兩個表中列為country的選出來並按照country排序
mysql> select country from website
-> union
-> select country from website
-> order by country;
+---------+
| country |
+---------+
| CN |
| IND |
| USA |
+---------+
3 rows in set (0.00 sec)
使用union all不刪除重複的資料
mysql> select country from website -> union all -> select country from app -> order by country; +---------+ | country | +---------+ | CN | | CN | | CN | | CN | | CN | | CN | | IND | | USA | | USA | +---------+ 9 rows in set (0.00 sec)
使用union合併使用where的select語句
mysql> select country, name from website where country = 'CN' -> union -> select country, app_name from app where country = 'CN'; +---------+--------------+ | country | name | +---------+--------------+ | CN | 淘寶 | | CN | 菜鳥教程 | | CN | 微博 | | CN | QQ APP | | CN | 微博 APP | | CN | 淘寶 APP | +---------+--------------+ 6 rows in set (0.00 sec)
2.排序語句order by：使用select語句可以查詢，可以使用order語句將查詢的結果排序後返回，可以選擇多個欄位進行排序，可以使用ASC或者DESC進行升序或者降序，預設是升序，根據alexa欄位進行排序的一個例子：
mysql> select * from website order by alexa asc;
+----+---------------+---------------------------+-------+---------+
| id | name | url | alexa | country |
+----+---------------+---------------------------+-------+---------+
| 6 | stackoverflow | http://stackoverflow.com/ | 0 | IND |
| 1 | Google | https://www.google.com/ | 1 | USA |
| 5 | Facebook | https://www.facebook.com/ | 3 | USA |
| 2 | 淘寶 | https:/www.taobao.com/ | 13 | CN |
| 4 | 微博 | http://weibo.com/ | 20 | CN |
| 3 | 菜鳥教程 | http://www.runoob.com | 4689 | CN |
+----+---------------+---------------------------+-------+---------+
6 rows in set (0.00 sec)
3.分組語句 group by：根據一個列或者多個對資料集進行分組，在分組的列上可以使用COUNT，SUM，AVG等函式，測試資料表如下：
mysql> select * from employee_tbl;
+----+--------+---------------------+--------+
| id | name | date | singin |
+----+--------+---------------------+--------+
| 1 | 小明 | 2016-04-22 15:25:33 | 1 |
| 2 | 小王 | 2016-04-20 15:25:47 | 3 |
| 3 | 小麗 | 2016-04-19 15:26:02 | 2 |
| 4 | 小王 | 2016-04-07 15:26:14 | 4 |
| 5 | 小明 | 2016-04-11 15:26:40 | 4 |
| 6 | 小明 | 2016-04-04 15:26:54 | 2 |
+----+--------+---------------------+--------+
6 rows in set (0.00 sec)
根據name進行分組並統計計數：
mysql> select name, count(*) from employee_tbl group by name;
+--------+----------+
| name | count(*) |
+--------+----------+
| 小麗 | 1 |
| 小明 | 3 |
| 小王 | 2 |
+--------+----------+
3 rows in set (0.00 sec)
根據name進行分組並計算singin之和：
mysql> select name, sum(singin) from employee_tbl group by name;
+--------+-------------+
| name | sum(singin) |
+--------+-------------+
| 小麗 | 2 |
| 小明 | 7 |
| 小王 | 7 |
+--------+-------------+
3 rows in set (0.00 sec)
使用with rollup語句對統計後的資料計算總數和，並使用as語句改變輸出的屬性值：
mysql> select name, sum(singin) as sum from employee_tbl group by name with rollup;
+--------+------+
| name | sum |
+--------+------+
| 小麗 | 2 |
| 小明 | 7 |
| 小王 | 7 |
| NULL | 16 |
+--------+------+
4 rows in set (0.00 sec)
這裡總數是NULL，可以指定名字，方法是使用coalesce函式，函式原型coalesce（a, b, c）若a是NULL，選擇b，若b是NULL，選擇c，如下：
mysql> select coalesce(name, '總數'), sum(singin) as sum_count from employee_tbl group by name with rollup;
+--------------------------+-----------+
| coalesce(name, '總數') | sum_count |
+--------------------------+-----------+
| 小麗 | 2 |
| 小明 | 7 |
| 小王 | 7 |
| 總數 | 16 |
+--------------------------+-----------+
4 rows in set (0.01 sec)
3.join語句：以上所有內容都是對於單個表的，union也是先查詢單張表然後查詢結果，join提供多表聯合查詢操作，可以在select、delete、update語句中使用。
join有3種：
inner join：內連線，獲取兩個表中欄位匹配的記錄
left join：左連線，獲取左表所有記錄，即使右表沒有對應匹配記錄
right join：右連線，獲取右表所有記錄，即使左表沒有對應匹配記錄
例如對如下兩張表，多表聯合查詢的含義就是，用runoob中的author欄位中的值，去tcount表中查runoob_count欄位的值
mysql> select * from runoob_tbl;
+-----------+---------------+---------------+-----------------+
| runoob_id | runoob_title | runoob_author | submission_date |
+-----------+---------------+---------------+-----------------+
| 1 | 學習 PHP | 菜鳥教程 | 2017-04-12 |
| 2 | 學習 MySQL | 菜鳥教程 | 2017-04-12 |
| 3 | 學習 Java | RUNOOB.COM | 2015-05-01 |
| 4 | 學習 Python | RUNOOB.COM | 2016-03-06 |
| 5 | 學習 C | FK | 2017-04-05 |
+-----------+---------------+---------------+-----------------+
5 rows in set (0.00 sec)
mysql> select * from tcount_tbl;
+---------------+--------------+
| runoob_author | runoob_count |
+---------------+--------------+
| 菜鳥教程 | 10 |
| RUNOOB.COM | 20 |
| Google | 22 |
+---------------+--------------+
3 rows in set (0.00 sec)使用inner join語句效果如下：
mysql> select a.runoob_id, a.runoob_author, b.runoob_count from runoob_tbl a inner join tcount_tbl b
-> on a.runoob_author = b.runoob_author;
+-----------+---------------+--------------+
| runoob_id | runoob_author | runoob_count |
+-----------+---------------+--------------+
| 1 | 菜鳥教程 | 10 |
| 2 | 菜鳥教程 | 10 |
| 3 | RUNOOB.COM | 20 |
| 4 | RUNOOB.COM | 20 |
+-----------+---------------+--------------+
等價於使用如下where語句：
mysql> select a.runoob_id, a.runoob_author, b.runoob_count from runoob_tbl a, tcount_tbl b where a.runoob_author = b.runoob_author;
+-----------+---------------+--------------+
| runoob_id | runoob_author | runoob_count |
+-----------+---------------+--------------+
| 1 | 菜鳥教程 | 10 |
| 2 | 菜鳥教程 | 10 |
| 3 | RUNOOB.COM | 20 |
| 4 | RUNOOB.COM | 20 |
+-----------+---------------+--------------+
4 rows in set (0.15 sec)
使用left join，會顯示左邊全部內容，即使在右表中沒有資料，沒有資料的屬性會顯示為NULL，如下
mysql> select a.runoob_id, a.runoob_author, b.runoob_count from runoob_tbl a left join tcount_tbl b
-> on a.runoob_author = b.runoob_author;
+-----------+---------------+--------------+
| runoob_id | runoob_author | runoob_count |
+-----------+---------------+--------------+
| 1 | 菜鳥教程 | 10 |
| 2 | 菜鳥教程 | 10 |
| 3 | RUNOOB.COM | 20 |
| 4 | RUNOOB.COM | 20 |
| 5 | FK | NULL |
+-----------+---------------+--------------+
使用right join，會顯示右表全部內容，在左表中沒有內容的屬性會顯示為NULL
mysql> select a.runoob_id, a.runoob_author, b.runoob_count from runoob_tbl a right join tcount_tbl b
-> on a.runoob_author = b.runoob_author;
+-----------+---------------+--------------+
| runoob_id | runoob_author | runoob_count |
+-----------+---------------+--------------+
| 1 | 菜鳥教程 | 10 |
| 2 | 菜鳥教程 | 10 |
| 3 | RUNOOB.COM | 20 |
| 4 | RUNOOB.COM | 20 |
| NULL | NULL | 22 |
+-----------+---------------+--------------+
4.NULL值的處理：mysql中的NULL值和程式語言中的NULL值比比較特殊，NULL和任何值比較都返回false，即使是NULL = NULL，因此不能用=和！=對NULL進行比較，mysql提供的專門的運算子解決這個問題，有3種：
IS NULL：當列的值是NULL，返回true
IS NOT NULL：當列的值不為NULL，返回true
<==>：當比較的兩個值都是NULL的時候，返回true
例如，對如下資料表進行操作，使用”=“判斷是否是NULL不生效
mysql> select * from runoob_test_tbl;
+---------------+--------------+
| runoob_author | runoob_count |
+---------------+--------------+
| RUNOOB | 20 |
| 菜鳥教程 | NULL |
| Google | NULL |
| FK | 20 |
+---------------+--------------+
4 rows in set (0.00 sec)
mysql> select * from runoob_test_tbl where runoob_count = NULL;
Empty set (0.01 sec)
mysql> select * from runoob_test_tbl where runoob_count is null;
+---------------+--------------+
| runoob_author | runoob_count |
+---------------+--------------+
| 菜鳥教程 | NULL |
| Google | NULL |
+---------------+--------------+