日期:2014-05-18 浏览次数:20505 次
DECLARE @dt datetime SET @dt = GETDATE() SELECT @dt, CAST(@dt AS binary(8)), --实际物理存储格式 CAST(@dt AS int), --datetime转为int是四舍五入 CAST(CAST(@dt AS decimal(18,9)) AS int), --decimal转为int是舍去小数 CAST(@dt AS decimal(18,9)), --datetime转为decimal CAST(SUBSTRING(CAST(@dt AS binary(8)),1,4) AS int) --分开日期和时间部分,分析datetime转为decimal的规则 + CAST(SUBSTRING(CAST(@dt AS binary(8)),5,8) AS int) * 1.0 / (24 * 60 * 60 * 300) --一秒中有300个10/3毫秒的间隔
SELECT sdt, dt1 = CAST(sdt AS datetime), --显式转换 dt2 = DATEADD(day,0,std) --隐式转换 FROM ( SELECT sdt = '' UNION --CAST('' AS datetime) == CAST(0 AS datetime) SELECT sdt = '01:02' UNION SELECT sdt = '01:02:03:004' UNION SELECT sdt = '20100521' UNION SELECT sdt = '2010-05-21' UNION SELECT sdt = '20100521 01:02' UNION SELECT sdt = '20100521 01:02:03' UNION SELECT sdt = '20100521 01:02:03.004' ) tmp
--以下查询对dtcolumn使用了函数,将不能使用索引 SELECT * FROM [table] WHERE DATEDIFF(day,dtcolumn,'20100826') = 0 --以下两种方法性能更好 SELECT * FROM [table] WHERE dtcolumn BETWEEN '20100826' AND '20100826 23:59:59' --只精确到秒,23:59:59之后一秒内的数据将无法查出 SELECT * FROM [table] WHERE dtcolumn >= '20100826' AND dtcolumn < '20100827' --完全精确,但需要把dtcolumn列名写两遍
SELECT 统计周期 = CONVERT(char(10),dtcolumn,120), 汇总 = COUNT(*) --或其它聚合函数 FROM [table] WHERE dtcolumn BETWEEN '20090101' AND '20101231 23:59:59' GROUP BY CONVERT(char(10),dtcolumn,120) ORDER BY 统计周期 --因为datetime按120转换为字符串的格式是2010-08-26 12:23:14,所以上面的CONVERT目标字符串长度改为4,7,10,13,16,19分别可以按年、月、日、时、分、秒统计。