sql函数coalesce和parse_url

学习函数系列：

coalesce

coalesce函数可以用来排除null值。
coalesce(a, b,c,d) 参数的个数没有限制
返回第一个参数中非null的值。

select

help coalesce\G;
[ 1. row ]
name | COALESCE
description | Syntax:
COALESCE(value,…)

Returns the first non-NULL value in the list, or NULL if there are no
non-NULL values.

The return type of COALESCE() is the aggregated type of the argument
types.

URL: https://dev.mysql.com/doc/refman/8.0/en/comparison-operators.html

example | mysql> SELECT COALESCE(NULL,1);
-> 1
mysql> SELECT COALESCE(NULL,NULL,NULL);
-> NULL

parse_url

if you want get some data from a url ,you can use the function parse_url。
it is a useful function。 you don’t need to write code by yourself（although, you can deal with url by regex）。
maybe this function inspired by PHP parse_url。
you can search hive parse_url，and learn how to use parse_url

replace

just like coding language, replace string
how to replace multiple values?

code sample:
SELECT REPLACE(‘abcdefghicde’,‘cde’,‘xxx’);

concat vs concat_ws

like strcat in c language
like + operator in python language
concat in sql in a function

concat_ws : you can get some information from its name ,CONCAT_WS indicates concatenate with separator