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                合規國際互聯網加速 OSASE為企業客戶提供高速穩定SD-WAN國際加速解決方案。 廣告
                # SQL Cookbook ## 查找空值 查找空值切記不能用`=`操作, 會返回`Empty Set` ~~~ mysql> select * from emp where comm is null; ~~~ ## 空值轉換為實際值返回 > 改變輸入的形式, 但并不改變表中的數據 coalesce()函數有1個或者多個參數, comm非空時返回comm, null時返回0, 也可以用`case語句判斷實現` ~~~ mysql> select coalesce(comm, 0) from emp; ~~~ ## 從一個表中查找與其他表不匹配的記錄 `外連接` ~~~ mysql> select d.* from dept d left outer join emp e -> on (d.deptno = e.deptno) where e.deptno is null; ~~~ ## 插入更新刪除 ## 插入技巧 ## 從一個表向另外的表中復制行 解決方案: 在insert語句后面緊跟一個用來產生索要插入行的查詢 ~~~ mysql> create table dept_east( -> deptno int, -> dname varchar(30), -> loc varchar(30)); mysql> insert into dept_east(deptno, dname, loc) -> select deptno, dname, loc from dept -> where loc in('new york', 'boston'); ~~~ ## 賦值表定義 > 只復制已有表的定義, 不復制其中的記錄, 創建表的時候, 使用`一個不返回任何行的子查詢, where的條件時鐘為false` ~~~ mysql> create table dept_2 -> as -> select * from dept where 1 = 0; ~~~ ## 修改技巧 ~~~ mysql> update emp set sal = sal * 1.10 where deptno = 20; ~~~ ## 刪除技巧 ### 刪除所有記錄 ~~~ mysql> delete from dept_2; ~~~
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