sql语法sql语法

SELECT emp_no FROM employees.salaries union select emp_no from employees.salaries;

EXPLAIN SELECT emp_no FROM employees.salaries union  select emp_no from employees.salaries;

EXPLAIN SELECT emp_no FROM employees.salaries union all select emp_no from employees.salaries;

select count(*) from employees.salaries;

explain select *  from (select * from employees.salaries) as Pp;

select cc.first_name, cc.salary from (select salaries.emp_no, salaries.salary, employees.first_name from employees.salaries join employees.employees on salaries.emp_no=employees.emp_no where salary > (select avg(salaries.salary) from employees.salaries)) as cc;

select avg(salaries.salary) from employees.salaries;

desc select salaries.emp_no, salaries.salary, employees.first_name from employees.salaries join employees.employees on salaries.emp_no=employees.emp_no;

explain select count(distinct salary) from employees.salaries;

show indexes from employees.salaries;
alter table employees.salaries add index keke(salary);
explain select distinct salary from employees.salaries  order by salary ;

show variables like 'slow%';

set global slow_query_log='ON'; 
set global slow_launch_time=1;
use employees;
CREATE TABLE employees.t1 (s1 INT, s2 CHAR(5), s3 FLOAT);

INSERT INTO employees.t1 VALUES (1,'1',1.0);
INSERT INTO employees.t1 VALUES (2,'2',2.0);
explain SELECT sb1,sb2,sb3
  FROM (SELECT s1 AS sb1, s2 AS sb2, s3*2 AS sb3 FROM employees.t1) AS sb
  WHERE sb1 > 1;

SELECT AVG(SUM(column1)) FROM t1 GROUP BY column1;

explain SELECT AVG(sum_column1)
  FROM (SELECT SUM(s1) AS sum_column1
        FROM t1 GROUP BY s1) AS t1;
--  没有用临时表
explain SELECT AVG(sum_column1)
  FROM (SELECT SUM(s1) AS sum_column1
        FROM t1) AS t1;