语法: COALESCE(T v1, T v2, …)
返回参数中的第一个非空值；如果所有值都为NULL，那么返回NULL
以emp表为例：
desc emp; empno int None
ename string None
job string None
mgr int None
hiredate string None
sal double None
comm double None
deptno int None
select * from emp; 7369 SMITH CLERK 7902 1980-12-17 800.0 NULL 20
7499 ALLEN SALESMAN 7698 1981-2-20 1600.0 300.0 30
7521 WARD SALESMAN 7698 1981-2-22 1250.0 500.0 30
7566 JONES MANAGER 7839 1981-4-2 2975.0 NULL 20
7654 MARTIN SALESMAN 7698 1981-9-28 1250.0 1400.0 30
7698 BLAKE MANAGER 7839 1981-5-1 2850.0 NULL 30
7782 CLARK MANAGER 7839 1981-6-9 2450.0 NULL 10
7788 SCOTT ANALYST 7566 1987-4-19 3000.0 NULL 20
7839 KING PRESIDENT NULL 1981-11-17 5000.0 NULL 10
7844 TURNER SALESMAN 7698 1981-9-8 1500.0 0.0 30
7876 ADAMS CLERK 7788 1987-5-23 1100.0 NULL 20
7900 JAMES CLERK 7698 1981-12-3 950.0 NULL 30
7902 FORD ANALYST 7566 1981-12-3 3000.0 NULL 20
7934 MILLER CLERK 7782 1982-1-23 1300.0 NULL 10
comm字段有空值（NULL）
select empno,ename,job,mgr,hiredate,sal, COALESCE(comm, 0),deptno from emp; 7369 SMITH CLERK 7902 1980-12-17 800.0 0.0 20
7499 ALLEN SALESMAN 7698 1981-2-20 1600.0 300.0 30
7521 WARD SALESMAN 7698 1981-2-22 1250.0 500.0 30
7566 JONES MANAGER 7839 1981-4-2 2975.0 0.0 20
7654 MARTIN SALESMAN 7698 1981-9-28 1250.0 1400.0 30
7698 BLAKE MANAGER 7839 1981-5-1 2850.0 0.0 30
7782 CLARK MANAGER 7839 1981-6-9 2450.0 0.0 10
7788 SCOTT ANALYST 7566 1987-4-19 3000.0 0.0 20
7839 KING PRESIDENT NULL 1981-11-17 5000.0 0.0 10
7844 TURNER SALESMAN 7698 1981-9-8 1500.0 0.0 30
7876 ADAMS CLERK 7788 1987-5-23 1100.0 0.0 20
7900 JAMES CLERK 7698 1981-12-3 950.0 0.0 30
7902 FORD ANALYST 7566 1981-12-3 3000.0 0.0 20
7934 MILLER CLERK 7782 1982-1-23 1300.0 0.0 10
注意观察comm字段的值。