mysql null string concatenation setting value +

# String concatenation concat (s1, s2); last_name string splicing table and in first_name

select concat(last_name,first_name) as  姓名 from employees;

# Only modifies first_name last_name is not modified

SELECT first_name,last_name AS f FROM employees;

# The two columns separated by commas and named out_put

SELECT CONCAT(`last_name`,',',`phone_number`) AS out_put FROM employees;

Determining whether #ifnull is empty, if blank is displayed instead of 0 null, and the column name displayed as the result

SELECT IFNULL(commission_pct,0) AS 结果 FROM employees;