MySQL: 组合查询，指定选取
HAVING
The HAVING clause was added to SQL because the WHERE keyword cannot be used with aggregate functions.
The following SQL statement lists the number of customers in each country, sorted high to low (Only include countries with more than 5 customers):
SELECT COUNT(CustomerID), Country
FROM Customers
GROUP BY Country
HAVING COUNT(CustomerID) > 5
ORDER BY COUNT(CustomerID) DESC;
重构
在重构table的时候（行转列、列转行）常用：
- UNION ALL
UNION ALL 可以将许多table纵向拼接
IS NOT NULL
使用在判断的地方，例如 WHERE price IS NOT NULL，可以作为一个判断是否为空的语句。
LIMIT and OFFSET
The LIMIT clause is used to specify the number of records to return.
The LIMIT clause is useful on large tables with thousands of records. Returning a large number of records can impact performance.
The following SQL statement selects the first three records from the "Customers" table:
SELECT * FROM Customers
LIMIT 3;
You can also specify an OFFSET from where to start returning data.
SELECT * FROM artists LIMIT 5 OFFSET [Number of rows to skip];
For example, if you want to get 5 artists, but not the first five. You want to get rows 3 through 8. You’ll want to add an OFFSET of 2 to skip the first two rows:
SELECT * FROM artists LIMIT 5 OFFSET 2;
About NULL
SELECT NULL will return NULL.