CSS 允许应用纯色作为背景，也允许使用背景图像创建相当复杂的效果。CSS 在这方面的能力远远在 HTML 之上
背景色
可以使用background-color 属性为元素设置背景色，这个属性接受任何合法的颜色值。
这条规则把元素的背景设为灰色：p{background-color :gray;}
如果你希望背景色从元素中的文本向外至少有延伸，只需增加一些内边框：p {background-color: gray; padding: 20px;}background-image 属性的默认值是 none，表示背景上没有放置任何图像。
背景图像
要把图像放入背景，需要使用 background-image 属性。
The function date_trunc is conceptually similar to the trunc function for numbers.
date_trunc('field', source)
source is a value expression of type timestamp or interval. (Values of type date and time are cast automatically, to timestamp or interval respectively.) field selects to which precision to truncate the input value. The return value is of type timestamp or interval with all fields that are less significant than the selected one set to zero (or one, for day and month).
Valid values for field are:
| microseconds |
| milliseconds |
| second |
| minute |
| hour |
| day |
| week |
| month |
| quarter |
| year |
| decade |
| century |
| millennium |
Examples:
SELECT date_trunc('hour', TIMESTAMP '2001-02-16 20:38:40');
Result: 2001-02-16 20:00:00
SELECT date_trunc('year', TIMESTAMP '2001-02-16 20:38:40');
Result: 2001-01-01 00:00:00