MySQL學習 (三) Limit-Distinct-Union

MySQL學習 (三) Limit-Distinct-Union

Limit

簡介

• 限制得到的記錄數量

mysql> SELECT * FROM teacher_class limit 1;
+----+--------+--------+-----------+------+------+------------+------------+
| id | t_name | gender | c_name    | room | days | begin_date | end_date   |
+----+--------+--------+-----------+------+------+------------+------------+
|  1 | Gene   | male   | python001 | 334  |   22 | 2013-03-31 | 2013-05-05 |
+----+--------+--------+-----------+------+------+------------+------------+
1 row in set (0.00 sec)

語法

Limit offset, row_count

# offset 偏移量，從0開始;
# row_count，總記錄數，長度，不是開始和結束, 若是數量大於餘下的記錄數，則獲取全部餘下的記錄數，默認值0

mysql> SELECT * FROM teacher_class limit 3,4;
+----+---------+--------+-----------+------+------+------------+------------+
| id | t_name  | gender | c_name    | room | days | begin_date | end_date   |
+----+---------+--------+-----------+------+------+------------+------------+
|  4 | Emma    | female | python004 | 338  |   20 | 2013-03-15 | 2013-04-05 |
|  5 | Janice  | female | python007 | 332  |   27 | 2013-04-15 | 2013-05-10 |
|  6 | Jessica | female | python008 | 332  |   28 | 2013-05-15 | 2013-06-08 |
+----+---------+--------+-----------+------+------+------------+------------+
3 rows in set (0.00 sec)

Distinct

簡介

• distinct: 去掉重複記錄， 指的是字段值，都相同的記錄，而不是部分字段相同的記錄
• All: 表示全部，默認就是all的行爲

mysql> SELECT days FROM teacher_class;
+------+
| days |
+------+
|   22 |
|   22 |
|   55 |
|   20 |
|   27 |
|   28 |
+------+
6 rows in set (0.00 sec)

mysql> SELECT DISTINCT days FROM teacher_class;
+------+
| days |
+------+
|   22 |
|   55 |
|   20 |
|   27 |
|   28 |
+------+
5 rows in set (0.00 sec)

mysql> SELECT days, begin_date FROM teacher_class;
+------+------------+
| days | begin_date |
+------+------------+
|   22 | 2013-03-31 |
|   22 | 2013-05-31 |
|   55 | 2013-02-15 |
|   20 | 2013-03-15 |
|   27 | 2013-04-15 |
|   28 | 2013-05-15 |
+------+------------+
6 rows in set (0.01 sec)

mysql> SELECT DISTINCT days, begin_date FROM teacher_class;
+------+------------+
| days | begin_date |
+------+------------+
|   22 | 2013-03-31 |
|   22 | 2013-05-31 |
|   55 | 2013-02-15 |
|   20 | 2013-03-15 |
|   27 | 2013-04-15 |
|   28 | 2013-05-15 |
+------+------------+
6 rows in set (0.00 sec)

union聯合查詢

簡介

• 將多條select語句的結果，合併到一塊兒，稱爲聯合

• 使用union關鍵字，聯合兩個select語句便可

• 注意 若是union查詢結果有重複，會自動去掉重複的數據；若是想獲得所有數據，經過UNION ALL來顯示數據

• 多個select語句檢索時，字段要一致; 數據類型，儘可能保持一致；若是不一致，應該能讓MySQL可以作類型轉換

• 檢索結果列名稱的問題：根據第一條select語句的字段，進行名稱排列

• 場景
  1. 得到數據的條件，出現邏輯衝突，或者很難在一個邏輯內表示，很差拆分，分紅多個邏輯，分別實現，而後合併到一塊兒

mysql> SELECT t_name, days FROM teacher_class WHERE c_name = "python002" ORDER BY days DESC LIMIT 1;
+---------+------+
| t_name  | days |
+---------+------+
| Jessica |   55 |
+---------+------+
1 row in set (0.00 sec)

mysql>
mysql> SELECT t_name, days FROM teacher_class WHERE c_name = "python008" ORDER BY days DESC LIMIT 1;
+---------+------+
| t_name  | days |
+---------+------+
| Jessica |   28 |
+---------+------+
1 row in set (0.00 sec)

# 聯合查詢語法

mysql>(SELECT t_name, days FROM teacher_class WHERE c_name = "python002" ORDER BY days DESC LIMIT 1) UNION (SELECT t_name, days FROM teacher_class WHERE c_name = "python008" ORDER BY days DESC LIMIT 1);
+---------+------+
| t_name  | days |
+---------+------+
| Jessica |   55 |
| Jessica |   28 |
+---------+------+
2 rows in set (0.00 sec)

子語句的結果排序

1. 將子語句包裹在括號裏
2. 子語句的order by須要使用limit才能生效，不然，排序失效

全部結果進行排序

1. 只須要在最後一個select子語句後，使用ORDER BY進行排序

mysql> (SELECT t_name, days FROM teacher_class WHERE c_name = "python002" ) \
    -> UNION (SELECT t_name, days FROM teacher_class WHERE c_name = "python008" ) ORDER BY days;
+---------+------+
| t_name  | days |
+---------+------+
| Mona    |   22 |
| Emma    |   28 |
| Jessica |   28 |
| Mona    |   55 |
| Jessica |   55 |
+---------+------+
5 rows in set (0.00 sec)