原创： 杨涛涛mysql

咱们知道，JSON是一种轻量级的数据交互的格式，大部分NO SQL数据库的存储都用JSON。MySQL从5.7开始支持JSON格式的数据存储，而且新增了不少JSON相关函数。MySQL 8.0 又带来了一个新的把JSON转换为TABLE的函数JSON_TABLE，实现了JSON到表的转换。sql

举例一

咱们看下简单的例子：数据库

简单定义一个两级JSON 对象json

mysql> set @ytt='{"name":[{"a":"ytt","b":"action"}, {"a":"dble","b":"shard"},{"a":"mysql","b":"oracle"}]}';

Query OK, 0 rows affected (0.00 sec)

第一级：oracle

mysql> select json_keys(@ytt);

+-----------------+

| json_keys(@ytt) |

+-----------------+

| ["name"] |

+-----------------+

1 row in set (0.00 sec)

第二级：函数

mysql> select json_keys(@ytt,'$.name[0]');

+-----------------------------+

| json_keys(@ytt,'$.name[0]') |

+-----------------------------+

| ["a", "b"] |

+-----------------------------+

1 row in set (0.00 sec)

咱们使用MySQL 8.0 的JSON_TABLE 来转换 @ytt。spa

mysql> select * from json_table(@ytt,'$.name[*]' columns (f1 varchar(10) path '$.a', f2 varchar(10) path '$.b')) as tt;

+-------+--------+

| f1 | f2 |

+-------+--------+

| ytt | action |

| dble | shard |

| mysql | oracle |

+-------+--------+

3 rows in set (0.00 sec)

举例二

再来一个复杂点的例子，用的是EXPLAIN 的JSON结果集。.net

JSON 串 @json_str1。code

set @json_str1 = ' {

"query_block": {

"select_id": 1,

"cost_info": {

"query_cost": "1.00"

},

"table": {

"table_name": "bigtable",

"access_type": "const",

"possible_keys": [

"id"

],

"key": "id",

"used_key_parts": [

"id"

],

"key_length": "8",

"ref": [

"const"

],

"rows_examined_per_scan": 1,

"rows_produced_per_join": 1,

"filtered": "100.00",

"cost_info": {

"read_cost": "0.00",

"eval_cost": "0.20",

"prefix_cost": "0.00",

"data_read_per_join": "176"

},

"used_columns": [

"id",

"log_time",

"str1",

"str2"

]

}

}

}';

第一级：对象

mysql> select json_keys(@json_str1) as 'first_object';

+-----------------+

| first_object |

+-----------------+

| ["query_block"] |

+-----------------+

1 row in set (0.00 sec)

第二级：

mysql> select json_keys(@json_str1,'$.query_block') as 'second_object';

+-------------------------------------+

| second_object |

+-------------------------------------+

| ["table", "cost_info", "select_id"] |

+-------------------------------------+

1 row in set (0.00 sec)

第三级：

mysql> select json_keys(@json_str1,'$.query_block.table') as 'third_object'\G

*************************** 1. row ***************************

third_object:

[

"key",

"ref",

"filtered",

"cost_info",

"key_length",

"table_name",

"access_type",

"used_columns",

"possible_keys",

"used_key_parts",

"rows_examined_per_scan",

"rows_produced_per_join"

]

1 row in set (0.01 sec)

第四级：

mysql> select json_extract(@json_str1,'$.query_block.table.cost_info') as 'forth_object'\G

*************************** 1. row ***************************

forth_object: {

"eval_cost":"0.20",

"read_cost":"0.00",

"prefix_cost":"0.00",

"data_read_per_join":"176"

}

1 row in set (0.00 sec)

那咱们把这个JSON 串转换为表。

SELECT * FROM JSON_TABLE(@json_str1,

"$.query_block"

COLUMNS(

rowid FOR ORDINALITY,

NESTED PATH '$.table'

COLUMNS (

a1_1 varchar(100) PATH '$.key',

a1_2 varchar(100) PATH '$.ref[0]',

a1_3 varchar(100) PATH '$.filtered',

nested path '$.cost_info'

columns (

a2_1 varchar(100) PATH '$.eval_cost' ,

a2_2 varchar(100) PATH '$.read_cost',

a2_3 varchar(100) PATH '$.prefix_cost',

a2_4 varchar(100) PATH '$.data_read_per_join'

),

a3 varchar(100) PATH '$.key_length',

a4 varchar(100) PATH '$.table_name',

a5 varchar(100) PATH '$.access_type',

a6 varchar(100) PATH '$.used_key_parts[0]',

a7 varchar(100) PATH '$.rows_examined_per_scan',

a8 varchar(100) PATH '$.rows_produced_per_join',

a9 varchar(100) PATH '$.key'

),

NESTED PATH '$.cost_info'

columns (

b1_1 varchar(100) path '$.query_cost'

),

c INT path "$.select_id"

)

) AS tt;

+-------+------+-------+--------+------+------+------+------+------+----------+-------+------+------+------+------+------+------+

| rowid | a1_1 | a1_2 | a1_3 | a2_1 | a2_2 | a2_3 | a2_4 | a3 | a4 | a5 | a6 | a7 | a8 | a9 | b1_1 | c |

+-------+------+-------+--------+------+------+------+------+------+----------+-------+------+------+------+------+------+------+

| 1 | id | const | 100.00 | 0.20 | 0.00 | 0.00 | 176 | 8 | bigtable | const | id | 1 | 1 | id | NULL | 1 |

| 1 | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | 1.00 | 1 |

+-------+------+-------+--------+------+------+------+------+------+----------+-------+------+------+------+------+------+------+

2 rows in set (0.00 sec)

固然，JSON_table 函数还有其余的用法，我这里不一一列举了，详细的参考手册。