check sentence:
select list of queries from table where the filter criteria;
Deduplication: select distinct query list ......
Select All: *
Since Alias: select list of queries as the table name from the alias
Filters:
First, according to the conditional expression screening:
Simple conditional operator:> <= = <!>> = <=
Second, the logical expression screening:
&& || ！
and or not
Third, the fuzzy query:
like：select * from employees where name like “王%”
(Means choosing employee information Wangs% represents any number of characters _ stands for any one character) To select _ and%, then later joined \
between and: concise language can increase the degree, and contains about value, value must be less than the value of the right to the left.
in: a field value determination whether an item in the list, in (......)
It is null: no query employee bonuses
select *
from employees
where pct is null；
Fourth, the security equal to <=> may substitute is null and =, but lower readability