| a ? b : c | case when athen b else c end | case when athen b else c end | case when athen b else c end | a成立时取b值，否则取c值 |
| a ?? b | ifnull(a, b) | isnull(a, b) | coalesce(a, b) | 当a为null时，取b值 |
| 数字 + 数字 | a + b | a + b | a + b | 数字相加 |
| 数字 + 字符串 | concat(a, b) | cast(a as varchar) + cast(b as varchar) | case(a as varchar)\|\| b | 字符串相加，a或b任意一个为字符串时 |
| a - b | a - b | a - b | a - b | 减
| a * b | a * b | a * b | a * b | 乘
| a / b | a / b | a / b | a / b | 乘
| a % b | a % b | a % b | a % b | 模
| a ? b : c | case when athen b else c end | case when athen b else c end | case when a then b else c end | case when a then b else c end | a成立时取b值，否则取c值 |
| a ?? b | ifnull(a, b) | isnull(a, b) | coalesce(a, b) | nvl(a, b) | 当a为null时，取b值 |
| 数字 + 数字 | a + b | a + b | a + b | a + b | 数字相加 |
| 数字 + 字符串 | concat(a, b) | cast(a as varchar) + cast(b as varchar) | case(a as varchar)\|\| b | a\|\| b | 字符串相加，a或b任意一个为字符串时 |
| string.IsNullOrEmpty(a) | (a is null or a = '') | (a is null or a = '') | (a is null or a = '') | (a is null or a = '') | 空字符串表示 |
| a.CompareTo(b) | strcmp(a, b) | - | case when a = b then 0 when a > b then 1 else -1 end | case when a = b then 0 when a > b then 1 else -1 end | 比较a和b大小 |
| a.Contains('b') | a like '%b%' | a like '%b%' | a ilike'%b%' | a like '%b%' | a是否包含b |
| a.EndsWith('b') | a like '%b' | a like '%b' | a ilike'%b' | a like '%b' | a尾部是否包含b |
| a.IndexOf(b) | locate(a, b) - 1 | locate(a, b) - 1 | strpos(a, b) - 1 | instr(a, b, 1, 1) - 1 | 查找a中出现b的位置 |
| DateTime.Compare(a, b) | a - b | a - b | extract(epoch from a::timestamp-b::timestamp) | extract(day from (a-b)) |
| DateTime.DaysInMonth(a, b) | dayofmonth(last_day(concat(a, '-', b, '-1'))) | datepart(day, dateadd(day, -1, dateadd(month, 1, cast(a as varchar) + '-' + cast(b as varchar) + '-1'))) | extract(day from (a || '-' || b || '-01')::timestamp+'1 month'::interval-'1 day'::interval) | cast(to_char(last_day(a||'-'||b||'-01'),'DD') as number) |
| DateTime.Equals(a, b) | a = b | a = b | a = b | a = b |
| DateTime.IsLeapYear(a) | a%4=0 and a%100<>0 or a%400=0 | a%4=0 and a%100<>0 or a%400=0 | a%4=0 and a%100<>0 or a%400=0 | mod(a,4)=0 AND mod(a,100)<>0 OR mod(a,400)=0 |
| DateTime.Parse(a) | cast(a as datetime) | cast(a as datetime) | a::timestamp | to_timestamp(a,'YYYY-MM-DD HH24:MI:SS.FF6') |
| a.Add(b) | date_add(a, interval b microsecond) | dateadd(millisecond, b / 1000, a) | a::timestamp+(b||' microseconds')::interval | 增加TimeSpan值 | a + b |
| a.AddDays(b) | date_add(a, interval b day) | dateadd(day, b, a) | a::timestamp+(b||' day')::interval | a + b |
| a.AddHours(b) | date_add(a, interval b hour) | dateadd(hour, b, a) | a::timestamp+(b||' hour')::interval | a + b/24 |
| a.AddMilliseconds(b) | date_add(a, interval b*1000 microsecond) | dateadd(millisecond, b, a) | a::timestamp+(b||' milliseconds')::interval | a + b/86400000 |
| a.AddMinutes(b) | date_add(a, interval b minute) | dateadd(minute, b, a) | a::timestamp+(b||' minute')::interval | a + b/1440 |
| a.AddMonths(b) | date_add(a, interval b month) | dateadd(month, b, a) | a::timestamp+(b||' month')::interval | add_months(a,b) |
| a.AddSeconds(b) | date_add(a, interval b second) | dateadd(second, b, a) | a::timestamp+(b||' second')::interval | a + b/86400 |
| a.AddTicks(b) | date_add(a, interval b/10 microsecond) | dateadd(millisecond, b / 10000, a) | a::timestamp+(b||' microseconds')::interval | a + b/86400000000 |
| a.AddYears(b) | date_add(a, interval b year) | dateadd(year, b, a) | a::timestamp+(b||' year')::interval | add_months(a,b*12) |
| a.Day | dayofmonth(a) | datepart(day, a) | extract(day from a::timestamp) | cast(to_char(a,'DD') as number) |
| a.DayOfWeek | dayofweek(a) | datepart(weekday, a) - 1 | extract(dow from a::timestamp) | case when to_char(a)='7' then 0 else cast(to_char(a) as number) end |
| a.DayOfYear | dayofyear(a) | datepart(dayofyear, a) | extract(doy from a::timestamp) | cast(to_char(a,'DDD') as number) |
| a.Hour | hour(a) | datepart(hour, a) | extract(hour from a::timestamp) | cast(to_char(a,'HH24') as number) |
| a.Millisecond | floor(microsecond(a) / 1000) | datepart(millisecond, a) | extract(milliseconds from a::timestamp)-extract(second from a::timestamp)*1000 | cast(to_char(a,'FF3') as number) |
| a.Minute | minute(a) | datepart(minute, a) | extract(minute from a::timestamp) | cast(to_char(a,'MI') as number) |
| a.Month | month(a) | datepart(month, a) | extract(month from a::timestamp) | cast(to_char(a,'FF3') as number) |
| a.Second | second(a) | datepart(second, a) | extract(second from a::timestamp) | cast(to_char(a,'SS') as number) |
| a.Subtract(b) | timestampdiff(microsecond, b, a) | datediff(millisecond, b, a) * 1000 | (extract(epoch from a::timestamp-b::timestamp)*1000000) | a - b |
| a.Ticks | timestampdiff(microsecond, '0001-1-1', a) * 10 | datediff(millisecond, '1970-1-1', a) * 10000 + 621355968000000000 | extract(epoch from a::timestamp)*10000000+621355968000000000 | cast(to_char(a,'FF7') as number) |
| a.TimeOfDay | timestampdiff(microsecond, date_format(a, '%Y-%m-%d'), a) | '1970-1-1 ' + convert(varchar, a, 14) | extract(epoch from a::time)*1000000 | a - trunc(a) |
| a.Year | year(a) | datepart(year, a) | extract(year from a::timestamp) | 年 | cast(to_char(a,'YYYY') as number) |
| a.Equals(b) | a = b | a = b | a = b | a = b |
| a.CompareTo(b) | a - b | a - b | a - b | a - b |
| TimeSpan.FromMilliseconds(a) | a * 1000 | a * 1000 | - | numtodsinterval(a/1000,'second') |
| TimeSpan.FromMinutes(a) | a * 1000000 * 60 | a * 1000000 * 60 | - | numtodsinterval(a*60,'second') |
| TimeSpan.FromSeconds(a) | a * 1000000 | a * 1000000 | - | numtodsinterval(a,'second') |
| TimeSpan.FromTicks(a) | a / 10 | a / 10 | - | numtodsinterval(a/10000000,'second') |
| a.Add(b) | a + b | a + b | - | a + b |
| a.Subtract(b) | a - b | a - b | - | a - b |
| a.CompareTo(b) | a - b | a - b | - | extract(day from (a-b)) |
| a.Days | a div (1000000 * 60 * 60 * 24) | a div (1000000 * 60 * 60 * 24) | - | extract(day from a) |
| a.Hours | a div (1000000 * 60 * 60) mod 24 | a div (1000000 * 60 * 60) mod 24 | - | extract(hour from a) |
| a.Milliseconds | a div 1000 mod 1000 | a div 1000 mod 1000 | - | cast(substr(extract(second from a)-floor(extract(second from a)),2,3) as number) |
| a.Seconds | a div 1000000 mod 60 | a div 1000000 mod 60 | - | extract(second from a) |
| a.Ticks | a * 10 | a * 10 | - | (extract(day from a)*86400+extract(hour from a)*3600+extract(minute from a)*60+extract(second from a))*10000000 |
| a.TotalDays | a / (1000000 * 60 * 60 * 24) | a / (1000000 * 60 * 60 * 24) | - | extract(day from a) |
| a.TotalHours | a / (1000000 * 60 * 60) | a / (1000000 * 60 * 60) | - | (extract(day from a)*24+extract(hour from a)) |
| a.TotalMilliseconds | a / 1000 | a / 1000 | - | (extract(day from a)*86400+extract(hour from a)*3600+extract(minute from a)*60+extract(second from a))*1000 |
| a.TotalMinutes | a / (1000000 * 60) | a / (1000000 * 60) | - | | (extract(day from a)*1440+extract(hour from a)*60+extract(minute from a)) |
| a.TotalSeconds | a / 1000000 | a / 1000000 | - | (extract(day from a)*86400+extract(hour from a)*3600+extract(minute from a)*60+extract(second from a)) |
| a.Equals(b) | a = b | a = b | - | a = b |
| a.ToString() | cast(a as varchar) | cast(a as varchar) | - | to_char(a) |
### 数学函数
| 表达式 | MySql | SqlServer | PostgreSQL |
| - | - | - | - |
| Math.Abs(a) | abs(a) | abs(a) | abs(a) |
| Math.Acos(a) | acos(a) | acos(a) | acos(a) |
| Math.Asin(a) | asin(a) | asin(a) | asin(a) |
| Math.Atan(a) | atan(a) | atan(a) | (atan(a)) |
| Math.Atan2(a, b) | atan2(a, b) | atan2(a, b) | atan2(a, b) |
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