1.Truncate
此函数用于返回X的截断到小数位D号的值。 如果D为0,则小数点被除去。如果D是负的,那么D的值的整数部分值的数量被截断。
-3表示有三位数要用0代替
2.PostgreSQL的generate_series函数应用
一、简介
PostgreSQL 中有一个很有用处的内置函数generate_series,可以按不同的规则产生一系列的填充数据。
二、语法
generate_series(start, stop, step)
生成一个数值序列,从start 到 stop,步进为step
例子
-- 设置bins的上下限,用generate_series设置start,end和步长
WITH bins AS (
SELECT generate_series(2200, 3050, 50) AS lower,
generate_series(2250, 3100, 50) AS upper),
-- subset stackoverflow to just tag dropbox
dropbox AS (
SELECT question_count
FROM stackoverflow
WHERE tag='dropbox')
-- select lower, upper, and count(*)来观察distribution
SELECT lower, upper, count(*)
-- from bins created above
FROM bins
-- join to dropbox and keep all rows from bins
LEFT JOIN dropbox
-- 重点来了,JOIN后可以接不等式!Compare question_count to lower and upper
ON question_count >= lower
AND question_count < upper
-- Group by lower and upper to count values in each bin
GROUP BY lower, upper
-- Order by lower to put bins in order
ORDER BY lower;