Aliases, de-duplication, the role of the +

Surnamed

benefit:

1, to facilitate the understanding

2, if the field you want to query the situation has the same name, an alias can be distinguished.

Method 1: Use as

select xingming as name;
select last_name as 姓,first_name as 名 from employees;

Second way: Use spaces

select last_name 姓，first_name 名 from employees;

Note: When there are among the alias keyword or spaces should be enclosed in double quotes to box up all aliases.

Deduplication

In the back of select add a keyword: distinct on it

select distinct department_id from employees;

The role of the +

In python "+" role is twofold:

1, the operator, the sum of two operands are numeric time

2, connector, when the two are strings, string concatenation

In mysql, + only one function: Operator

①: both operands are numeric, do adder

②: wherein one character is trying to convert the character into a numerical vertical, if the conversion is successful (string of digital content), then continue to do adder; cleavage conversion fails, then converted to a character value 0.

③: As long as one party is null, null is sure to get results