MySQL DML之常見函式
常見函式
概念:類似於java的方法,將一組邏輯語句封裝在方法體中,對外暴露方法名
好處:
- 1、隱藏了實現細節
- 2、提高程式碼的重用性
呼叫:select 函式名(實參列表) 【from 表】;
特點:
①叫什麼(函式名)
②幹什麼(函式功能)分類:
1、單行函式
如 concat、length、ifnull等
2、分組函式
功能:做統計使用,又稱為統計函式、聚合函式、組函式
常見函式:
一、單行函式
字元函式:
- length:獲取位元組個數(utf-8一個漢字代表3個位元組,gbk為2個位元組)
- concat
- substr
- instr
- trim
- upper
- lower
- lpad
- rpad
- replace
數學函式:
- round
- ceil
- floor
- truncate
- mod
日期函式:
- now
- curdate
- curtime
- year
- month
- monthname
- day
- hour
- minute
- second
- str_to_date
- date_format
其他函式:
- version
- database
- user
控制函式
if case
一、字元函式
1. length 獲取引數值的位元組個數
SELECT LENGTH('john');
SELECT LENGTH('張三丰hahaha');
SHOW VARIABLES LIKE '%char%'
2. concat 拼接字串
SELECT CONCAT(last_name,'_',first_name) 姓名 FROM employees;
3. upper、lower
SELECT UPPER('john'); SELECT LOWER('joHn');
示例:將姓變大寫,名變小寫,然後拼接;
SELECT CONCAT(UPPER(last_name),LOWER(first_name)) 姓名 FROM employees;
4. substr、substring(注意:索引從1開始)
案例一:擷取從指定索引處後面所有字元
SELECT SUBSTR('李莫愁愛上了陸展元',7) out_put;
案例二:擷取從指定索引處指定字元長度的字元
SELECT SUBSTR('李莫愁愛上了陸展元',1,3) out_put;
案例三:姓名中首字元大寫,其他字元小寫然後用_拼接,顯示出來
SELECT CONCAT(UPPER(SUBSTR(last_name,1,1)),'_',LOWER(SUBSTR(last_name,2))) out_put
FROM employees;
5. instr 返回子串第一次出現的索引,如果找不到返回0
SELECT INSTR('楊不殷六俠悔愛上了殷六俠','殷八俠') AS out_put;
6. trim
SELECT LENGTH(TRIM(' 張翠山 ')) AS out_put;
SELECT TRIM('aa' FROM 'aaaaaaaaa張aaaaaaaaaaaa翠山aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa') AS out_put;
輸出結果:
mysql> SELECT LENGTH(TRIM(' 張翠山 ')) AS out_put;
+---------+
| out_put |
+---------+
| 6 |
+---------+
1 row in set (0.00 sec)
mysql> SELECT TRIM('aa' FROM 'aaaaaaaaa張aaaaaaaaaaaa翠山aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa') AS out_put;
+----------------------+
| out_put |
+----------------------+
| a張aaaaaaaaaaaa翠山a |
+----------------------+
1 row in set (0.00 sec)
7. lpad 用指定的字元實現左填充指定長度
SELECT LPAD('殷素素',2,'*') AS out_put;
輸出結果:
mysql> SELECT LPAD('殷素素',10,'*') AS out_put;
+---------------+
| out_put |
+---------------+
| *******殷素素 |
+---------------+
1 row in set (0.00 sec)
8. rpad 用指定的字元實現右填充指定長度
SELECT RPAD('殷素素',12,'ab') AS out_put;
輸出結果:
mysql> SELECT RPAD('殷素素',12,'ab') AS out_put;
+-----------------+
| out_put |
+-----------------+
| 殷素素ababababa |
+-----------------+
1 row in set (0.00 sec)
9. replace 替換
SELECT REPLACE('周芷若周芷若周芷若周芷若張無忌愛上了周芷若','周芷若','趙敏') AS out_put;
輸出結果:
mysql> SELECT REPLACE('周芷若周芷若周芷若周芷若張無忌愛上了周芷若','周芷若','趙敏') AS out_put;
+----------------------------------+
| out_put |
+----------------------------------+
| 趙敏趙敏趙敏趙敏張無忌愛上了趙敏 |
+----------------------------------+
1 row in set (0.00 sec)
二、數學函式
1. round 四捨五入
SELECT ROUND(-1.55);
SELECT ROUND(1.567,2);
輸出結果:
mysql> select round(1.56);
+-------------+
| round(1.56) |
+-------------+
| 2 |
+-------------+
1 row in set (0.00 sec)
mysql> select round(-1.56);
+--------------+
| round(-1.56) |
+--------------+
| -2 |
+--------------+
1 row in set (0.00 sec)
mysql> select round(1.567, 2);
+-----------------+
| round(1.567, 2) |
+-----------------+
| 1.57 |
+-----------------+
1 row in set (0.00 sec)
對於負數,可將其先堪稱整數四捨五入,然後再加上負號;
2. ceil 向上取整,返回>=該引數的最小整數;
SELECT CEIL(-1.02);
輸出結果:
mysql> SELECT CEIL(-1.02);
+-------------+
| CEIL(-1.02) |
+-------------+
| -1 |
+-------------+
1 row in set (0.00 sec)
3. floor 向下取整,返回<=該引數的最大整數
SELECT FLOOR(-9.99);
輸出結果:
mysql> SELECT FLOOR(-9.99);
+--------------+
| FLOOR(-9.99) |
+--------------+
| -10 |
+--------------+
1 row in set (0.00 sec)
4. truncate 截斷
SELECT TRUNCATE(1.69999,1);
輸出結果:
mysql> SELECT TRUNCATE(1.69999,1);
+---------------------+
| TRUNCATE(1.69999,1) |
+---------------------+
| 1.6 |
+---------------------+
1 row in set (0.00 sec)
5. mod取餘
SELECT MOD(10,-3);
SELECT 10%3;
輸出結果:
mysql> SELECT MOD(10,-3);
+------------+
| MOD(10,-3) |
+------------+
| 1 |
+------------+
1 row in set (0.00 sec)
mysql> SELECT 10%3;
+------+
| 10%3 |
+------+
| 1 |
+------+
1 row in set (0.00 sec)
三、日期函式
1. now 返回當前系統日期+時間
SELECT NOW();
輸出結果:
mysql> select now();
+---------------------+
| now() |
+---------------------+
| 2018-11-17 10:39:18 |
+---------------------+
1 row in set (0.00 sec)
2. curdate 返回當前系統日期,不包含時間
SELECT CURDATE();
輸出結果:
mysql> select curdate();
+------------+
| curdate() |
+------------+
| 2018-11-17 |
+------------+
1 row in set (0.00 sec)
3. curtime 返回當前時間,不包含日期
SELECT CURTIME();
輸出結果:
mysql> SELECT CURTIME();
+-----------+
| CURTIME() |
+-----------+
| 10:41:02 |
+-----------+
1 row in set (0.00 sec)
4. 可以獲取指定的部分,年、月、日、小時、分鐘、秒
SELECT YEAR(NOW()) 年;
SELECT YEAR('1998-1-1') 年;
SELECT YEAR(hiredate) 年 FROM employees;
SELECT MONTH(NOW()) 月;
SELECT MONTHNAME(NOW()) 月;
輸出結果:
mysql> SELECT YEAR(NOW()) 年;
+------+
| 年 |
+------+
| 2018 |
+------+
1 row in set (0.00 sec)
mysql> SELECT YEAR('1998-1-1') 年;
+------+
| 年 |
+------+
| 1998 |
+------+
1 row in set (0.00 sec)
mysql> SELECT DISTINCT YEAR(hiredate) 年 FROM employees;
+------+
| 年 |
+------+
| 1992 |
| 1998 |
| 2000 |
| 2004 |
| 2002 |
| 2014 |
| 2016 |
+------+
7 rows in set (0.00 sec)
mysql> SELECT MONTH(NOW()) 月;
+------+
| 月 |
+------+
| 11 |
+------+
1 row in set (0.00 sec)
mysql> SELECT MONTHNAME(NOW()) 月;
+----------+
| 月 |
+----------+
| November |
+----------+
1 row in set (0.00 sec)
5. str_to_date 將字元通過指定的格式轉換成日期
SELECT STR_TO_DATE('1998-3-2','%Y-%c-%d') AS out_put;
輸出結果:
mysql> SELECT STR_TO_DATE('1998-3-2','%Y-%c-%d') AS out_put;
+------------+
| out_put |
+------------+
| 1998-03-02 |
+------------+
1 row in set (0.00 sec)
案例:查詢入職日期為1992--4-3的員工資訊
SELECT * FROM employees WHERE hiredate = STR_TO_DATE('4-3 1992','%c-%d %Y');
輸出結果:
mysql> SELECT * FROM employees WHERE hiredate = STR_TO_DATE('4-3 1992','%c-%d %Y');
+-------------+------------+-----------+----------+--------------+---------+----------+----------------+------------+---------------+---------------------+
| employee_id | first_name | last_name | email | phone_number | job_id | salary | commission_pct | manager_id | department_id | hiredate |
+-------------+------------+-----------+----------+--------------+---------+----------+----------------+------------+---------------+---------------------+
| 100 | Steven | K_ing | SKING | 515.123.4567 | AD_PRES | 24000.00 | NULL | NULL | 90 | 1992-04-03 00:00:00 |
| 101 | Neena | Kochhar | NKOCHHAR | 515.123.4568 | AD_VP | 17000.00 | NULL | 100 | 90 | 1992-04-03 00:00:00 |
| 102 | Lex | De Haan | LDEHAAN | 515.123.4569 | AD_VP | 17000.00 | NULL | 100 | 90 | 1992-04-03 00:00:00 |
| 103 | Alexander | Hunold | AHUNOLD | 590.423.4567 | IT_PROG | 9000.00 | NULL | 102 | 60 | 1992-04-03 00:00:00 |
| 104 | Bruce | Ernst | BERNST | 590.423.4568 | IT_PROG | 6000.00 | NULL | 103 | 60 | 1992-04-03 00:00:00 |
+-------------+------------+-----------+----------+--------------+---------+----------+----------------+------------+---------------+---------------------+
5 rows in set (0.00 sec)
6. date_format 將日期轉換成字元
SELECT DATE_FORMAT(NOW(),'%y年%m月%d日') AS out_put;
輸出結果:
mysql> SELECT DATE_FORMAT(NOW(),'%y年%m月%d日') AS out_put;
+--------------+
| out_put |
+--------------+
| 18年11月17日 |
+--------------+
1 row in set (0.00 sec)
7. curtime 返回當前時間,不包含日期
SELECT CURTIME();
輸出結果:
mysql> SELECT CURTIME();
+-----------+
| CURTIME() |
+-----------+
| 10:41:02 |
+-----------+
1 row in set (0.00 sec)
四、其他函式
SELECT VERSION();
SELECT DATABASE();
SELECT USER();
五、流程控制函式
1. if函式: if else 的效果
SELECT IF(10<5,'大','小');
輸出結果:
mysql> SELECT IF(10>5,'大','小');
+--------------------+
| IF(10>5,'大','小') |
+--------------------+
| 大 |
+--------------------+
1 row in set (0.00 sec)
2. case函式的使用一: switch case 的效果(常用於常量)
java中
switch(變數或表示式){
case 常量1:語句1;break;
...
default:語句n;break;
}mysql中
case 要判斷的欄位或表示式
when 常量1 then 要顯示的值1或語句1;
when 常量2 then 要顯示的值2或語句2;
...
else 要顯示的值n或語句n;
end
案例:查詢員工的工資,要求
部門號=30,顯示的工資為1.1倍
部門號=40,顯示的工資為1.2倍
部門號=50,顯示的工資為1.3倍
其他部門,顯示的工資為原工資
select salary 原始工資, department_id,
case department_id
when 30 then salary * 1.1
when 40 then salary * 1.2
when 50 then salary * 1.3
else salary
end as 新工資
from employees;
輸出結果:
mysql> select salary 原始工資, department_id,
-> case department_id
-> when 30 then salary * 1.1
-> when 40 then salary * 1.2
-> when 50 then salary * 1.3
-> else salary
-> end as 新工資
-> from employees;
+----------+---------------+----------+
| 原始工資 | department_id | 新工資 |
+----------+---------------+----------+
| 24000.00 | 90 | 24000.00 |
| 17000.00 | 90 | 17000.00 |
| 17000.00 | 90 | 17000.00 |
| 9000.00 | 60 | 9000.00 |
| 6000.00 | 60 | 6000.00 |
| 4800.00 | 60 | 4800.00 |
3.case 函式的使用二:類似於 多重if(常適用於範圍)
java中:
if(條件1){
語句1;
}else if(條件2){
語句2;
}
...
else{
語句n;
}mysql中:
case
when 條件1 then 要顯示的值1或語句1
when 條件2 then 要顯示的值2或語句2
。。。
else 要顯示的值n或語句n
end
SELECT salary,
CASE
WHEN salary > 20000 THEN 'A'
WHEN salary > 15000 THEN 'B'
WHEN salary > 10000 THEN 'C'
ELSE 'D'
END AS 工資級別
FROM employees;
輸出結果:
mysql> SELECT salary,
-> CASE
-> WHEN salary > 20000 THEN 'A'
-> WHEN salary > 15000 THEN 'B'
-> WHEN salary > 10000 THEN 'C'
-> ELSE 'D'
-> END AS 工資級別
-> FROM employees;
+----------+----------+
| salary | 工資級別 |
+----------+----------+
| 24000.00 | A |
| 17000.00 | B |
| 17000.00 | B |
| 9000.00 | D |