文章目录

• 1 概述
• 2 示例
• • 2.1 isnull(expr)：expr 是否为空
  • 2.2 ifnull(expr1, expr2)：expr1 是否为空
• 3 扩展
• • 3.1 nullif(expr1, expr2)：俩表达式是否相等
  • 3.2 if(expr1, value1, value2)：expr1 是否成立
  • 3.3 coalesce(value1, valueN)：返回第一个非空值

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-- 判断 expr 是否为空
---- 若是，返回 1
---- 若否，返回 0
isnull(expr): return 1 or 0

select isnull(null) 是, -- 1isnull('a')  否, -- 0isnull('')   否  -- 0from dual;

-- 判断 expr1 是否为空
---- 若是，返回 expr2
---- 若否，返回 expr1
ifnull(expr1, expr2): return expr2 or expr1

select ifnull(null, '2'),  -- '2'ifnull('1', null),  -- '1'ifnull(null, null)  -- nullfrom dual;

-- 判断 expr1 = expr2 是否成立
---- 若是，返回 null
---- 若否，返回 expr1
nullif(expr1, expr2)：return null or expr1

select nullif('1', '1'),  -- nullnullif('1', '2')   -- '1'from dual;

-- 判断 expr1 是否为 True
---- 若是，返回 value1
---- 若否，返回 value2
-- 小提示：0：False，非0：True
if(expr, value1, value2): return value1 or value2

select if(1 = 1, '是', '否'),  -- 是if(1 = 2, '是', '否')   -- 否from dual;

-- 返回多个值中，第一个非空的值
select coalesce(null, null, '1', '2')  -- '1'from dual;