mysql索引分析之一
优化 SQL 的最高境界是回归需求，只有正确的理解了需求，才能写出高效的SQL，因此改写 SQL 往往是优化SQL 的重要手段。
虽然说读懂执行计划是SQL 优化中极其重要的环节，但是如果不能读懂需求，写出复杂繁琐绕了九曲十八弯的SQL 语句，通过分析执行计划来调优，往往还是会陷入到束手无策之中。
1、join查询类型
1.1、SQL书写顺序
SELECT <select_list>
FROM
<left_table> <join_type> JOIN <right_table> ON <join_condition>
WHERE <where_condition>
GROUP BY <group_by_list>
HAVING <having_condition>
ORDER BY <order_by_condition>
LIMIT <limit_number>
1.2、SQL执行顺序
FROM <left_table>
ON <join_condition>
<join_type> JOIN <right_table>
WHERE <where_condition>
GROUP BY <group_by_list>
HAVING <having_condition>
SELECT
DISTINCT <select_list>
ORDER BY <order_by_condition>
LIMIT <limit_number>
1.3、SQL执行结果小结
2 JOIN查询结果集图解
2.1 LEFT JOIN ON
SELECT <select_list>
FROM tableA A
LEFT JOIN tableB B
ON A.key = B.key;
2.2 RIGHT JOIN ON
SELECT <select_list>
FROM tableA A
RIGHT JOIN tableB B
ON A.key = B.key;
2.3 INNER JOIN ON
join 默认为 inner join
SELECT <select_list>
FROM tableA A
INNER JOIN tableB B
ON A.key = B.key;
2.4 LEFT JOIN ON WHERE
SELECT <select_list>
FROM tableA A
LEFT JOIN tableB B
ON A.key = B.key
WHERE B.key IS NULL;
2.5 RIGHT JOIN ON WHERE
SELECT <select_list>
FROM tableA A
RIGHT JOIN tableB B
ON A.key = B.key
WHERE A.key IS NULL;
2.6 FULL OUTER JOIN ON
SELECT <select_list>
FROM tableA A
FULL OUTER JOIN tableB B
ON A.key = B.key
2.7 FULL OUTER JOIN ON WHERE
SELECT <select_list>
FROM tableA A
FULL OUTER JOIN tableB B
ON A.key = B.key
WHERE A.key IS NULL OR
B.key IS NULL
2.3、建表SQL
2.4、7种join实例
3、索引介绍
4、性能分析
5、索引优化
1、性能下降SQL慢、执行时间长、等待时间长