1.convert(value,type);
将value转换为指定的类型b；
sql语句要以分号结尾，通过向上键选择想要直接使用或修改较少的sql语句。
2.cast(value as type);
3.ifnull(a,b);
select ifnull(a,b);如果a不为null,则选择a,反之则选择b；
4.limit a,b
从第a个开始，找b个值
5.top number/percent
找最前边的number个值
6. @变量名，:=进行赋值，且需要在from字句中进行初始化
SELECT Score, CONVERT (
CASE
WHEN @preValue = Score THEN @preRank
WHEN (@preValue := Score) >= 0 THEN @preRank := @preRank + 1
END,
UNSIGNED) AS Rank
FROM Scores, (SELECT @preValue := NULL) p, (SELECT @preRank := 0) v
ORDER BY Score DESC;
7.select Score,(select count(distinct Score )from Scores as s2 where s2.Score>=s1.Score) Rank
from Scores as s1
order by Score DESC;
8.排名一定要使用distinct，相同的值属于同一排序值。
9.sql执行顺序
标准的 SQL 的解析顺序为:
(1) FROM 子句 组装来自不同数据源的数据
(2) WHERE 子句 基于指定的条件对记录进行筛选
(3) GROUP BY 子句 将数据划分为多个分组
(4) 使用聚合函数进行计算
(5) 使用HAVING子句筛选分组
(6) 计算所有的表达式
(7) 使用ORDER BY对结果集进行排序
10.set @cnt := 0, @lnum := -101;
select distinct t.Num ConsecutiveNums from (select Num, (case when Num <> @lnum then @cnt := 1 else @cnt := @cnt+1 end) as cnt, @lnum := Num from Logs)t where t.cnt >=3;
11.SELECT D1.Name Department, E1.Name Employee, E1.Salary
FROM Employee E1, Employee E2, Department D1
WHERE E1.DepartmentID = E2.DepartmentID
AND E2.Salary >= E1.Salary
AND E1.DepartmentID = D1.ID
GROUP BY E1.Name
HAVING COUNT(DISTINCT E2.Salary) <= 3
ORDER BY D1.Name, E1.Salary DESC;
12.delete from person where id not in (
select id
from (select min(id) as id from person group by email) p)