MYSQL學習:基礎查詢:去重,+號用法,連線字串
阿新 • • 發佈:2021-01-09
基礎查詢
1.去重:DISTINCT(distinct)
語法:select DISTINCT 查詢欄位 FROM 表;
#案例 mysql> select distinct department_id from employees; +---------------+ | department_id | +---------------+ | NULL | | 10 | | 20 | | 30 | | 40 | | 50 | +---------------+ 12 rows in set (0.06 sec)
2. +號的作用
有且僅有一個功能:運算子
只要其中一方為null,則結果一定為null(select null+100;)結果為null
**兩個運算元都為數值型,做加法運算** select 100+90; +--------+ | 100+90 | +--------+ | 190 | +--------+ **其中一方為字元型,則試圖轉換字元型為數值型,成功則做加法運算; 失敗則將字元型數值轉換為0** mysql> select '87'+67; +---------+ | '87'+67 | +---------+ | 154 | +---------+ mysql> select 'hello'+21; +------------+ | 'hello'+21 | +------------+ | 21 | +------------+
3.連線字元:concat
語法:select concat(str1,str2,…) from 表名;
**案例**
mysql>select concat(last_name,first_name) as 姓名 from employees;
**4.練習
顯示出表 employees 的全部列,各個列之間用逗號連線,列頭顯示成 OUT_PUT **
**引號中的逗號,也會執行查詢!** mysql> select concat(first_name,',',last_name,',',job_id) as out_put -> from employees; +------------------------------+ | out_put | +------------------------------+ | Steven,K_ing,AD_PRES | | Neena,Kochhar,AD_VP |