select查询语句执行顺序
- 开始-->from子句-->where子句-->group by子句-->having子句-->order by子句-->select子句-->limit子句-->最总结果
* from（从）从那张表
* where（哪里）有条件地从表中选取数据
* group by（通过...分组）语句用于结合合计函数，根据一个或多个列对结果集进行分组
* having（有） 在 SQL 中增加 HAVING 子句原因是，WHERE 关键字无法与合计函数一起使用
* order by（排序）用于对结果集进行排序，默认按照升序对记录进行排序
* select（选择）语句用于从表中选取数据，结果被存储在一个结果表中（称为结果集）
* limit（限制）
最简单的查询语句
MariaDB [(none)]> select 1;
+---+
| 1 |
+---+
| 1 |
+---+
1 row in set (0.00 sec)
对查询结果做投影，字段名未找到也是1 ，尝试select 1 as a；
MariaDB [test]> select * from employees;
+--------+------------+------------+-------------+--------+------------+
| emp_no | birth_date | first_name | last_name | gender | hire_date |
+--------+------------+------------+-------------+--------+------------+
| 10001 | 1953-09-02 | Georgi | Facello | M | 1986-06-26 |
| 10002 | 1964-06-02 | Bezalel | Simmel | F | 1985-11-21 |
| 10003 | 1959-12-03 | Parto | Bamford | M | 1986-08-28 |
| 10004 | 1954-05-01 | Chirstian | Koblick | M | 1986-12-01 |
| 10005 | 1955-01-21 | Kyoichi | Maliniak | M | 1989-09-12 |
| 10006 | 1953-04-20 | Anneke | Preusig | F | 1989-06-02 |
| 10007 | 1957-05-23 | Tzvetan | Zielinski | F | 1989-02-10 |
| 10008 | 1958-02-19 | Saniya | Kalloufi | M | 1994-09-15 |
| 10009 | 1952-04-19 | Sumant | Peac | F | 1985-02-18 |
| 10010 | 1963-06-01 | Duangkaew | Piveteau | F | 1989-08-24 |
| 10011 | 1953-11-07 | Mary | Sluis | F | 1990-01-22 |
| 10012 | 1960-10-04 | Patricio | Bridgland | M | 1992-12-18 |
| 10013 | 1963-06-07 | Eberhardt | Terkki | M | 1985-10-20 |
| 10014 | 1956-02-12 | Berni | Genin | M | 1987-03-11 |
| 10015 | 1959-08-19 | Guoxiang | Nooteboom | M | 1987-07-02 |
| 10016 | 1961-05-02 | Kazuhito | Cappelletti | M | 1995-01-27 |
| 10017 | 1958-07-06 | Cristinel | Bouloucos | F | 1993-08-03 |
| 10018 | 1954-06-19 | Kazuhide | Peha | F | 1987-04-03 |
| 10019 | 1953-01-23 | Lillian | Haddadi | M | 1999-04-30 |
| 10020 | 1952-12-24 | Mayuko | Warwick | M | 1991-01-26 |
+--------+------------+------------+-------------+--------+------------+
20 rows in set (0.00 sec)
最常见的查询语句，查询这个表中的所有字段