一、JSON_EXTRACT函数的语法
JSON_EXTRACT函数的语法如下:JSON_EXTRACT(json_doc, path[, path] ...)其中,json_doc是要提取数据的JSON文档,而path则是要提取的数据路径,可以是一个或多个路径参数。path参数还支持字符串函数,如CONCAT和SUBSTRING,这些函数可以与路径参数一起使用。
二、JSON_EXTRACT函数的示例
让我们通过几个示例来进一步了解JSON_EXTRACT函数的使用。1. 从JSON数组中提取元素:可以使用以下命令从JSON数组中提取第一个元素:
SELECT JSON_EXTRACT('[1, 2, 3]', '$[0]'); -- 返回值为1
2. 从嵌套的JSON对象中提取值:假设我们有以下JSON对象:
{ "name": "John", "age": 30, "address": { "street": "123 Main St", "city": "Anytown", "state": "TX", "zip": "12345" } }
我们可以使用以下命令从中提取州名称:
SELECT JSON_EXTRACT('{ "name": "John", "age": 30, "address": { "street": "123 Main St", "city": "Anytown", "state": "TX", "zip": "12345" } }', '$.address.state'); -- 返回值为"TX"
3. 提取嵌套的JSON数组:假设我们有以下JSON对象:
{ "store": { "book": [ { "category": "reference", "author": "Nigel Rees", "title": "Sayings of the Century", "price": 8.95 }, { "category": "fiction", "author": "Evelyn Waugh", "title": "Sword of Honour", "price": 12.99 }, { "category": "fiction", "author": "Herman Melville", "title": "Moby Dick", "isbn": "0-553-21311-3", "price": 8.99 }, { "category": "fiction", "author": "J. R. R. Tolkien", "title": "The Lord of the Rings", "isbn": "0-395-19395-8", "price": 22.99 } ] } }
我们可以使用以下命令从中提取所有书籍的价格:
SELECT JSON_EXTRACT('{ "store": { "book": [ { "category": "reference", "author": "Nigel Rees", "title": "Sayings of the Century", "price": 8.95 }, { "category": "fiction", "author": "Evelyn Waugh", "title": "Sword of Honour", "price": 12.99 }, { "category": "fiction", "author": "Herman Melville", "title": "Moby Dick", "isbn": "0-553-21311-3", "price": 8.99 }, { "category": "fiction", "author": "J. R. R. Tolkien", "title": "The Lord of the Rings", "isbn": "0-395-19395-8", "price": 22.99 } ] } }', '$.store.book[*].price');该命令将返回一个JSON数组,其中包含所有书籍的价格。
三、使用JSON_EXTRACT函数进行数据过滤
除了从JSON文档中提取数据,JSON_EXTRACT函数还能用于进行数据过滤。通过在WHERE子句中使用JSON_EXTRACT函数,我们可以仅查询符合特定条件的记录。 假设我们有以下JSON文档:{ "students": [ { "name": "John Doe", "gender": "male", "age": 18, "graduated": false, "scores": [ { "subject": "math", "score": 98 }, { "subject": "history", "score": 88 } ] }, { "name": "Jane Doe", "gender": "female", "age": 19, "graduated": true, "scores": [ { "subject": "math", "score": 78 }, { "subject": "history", "score": 92 } ] } ] }我们可以使用以下命令查询出年龄大于18岁且数学分数大于90分的学生:
SELECT JSON_EXTRACT('{ "students": [ { "name": "John Doe", "gender": "male", "age": 18, "graduated": false, "scores": [ { "subject": "math", "score": 98 }, { "subject": "history", "score": 88 } ] }, { "name": "Jane Doe", "gender": "female", "age": 19, "graduated": true, "scores": [ { "subject": "math", "score": 78 }, { "subject": "history", "score": 92 } ] } ] }', '$.students[?(@.age > 18 && @.scores[?(@.subject == "math")].score > 90)]');该命令将返回一个JSON数组,其中包含所有符合条件的学生信息。
四、使用CONCAT和SUBSTRING函数进行数据处理
我们可以在JSON_EXTRACT函数中使用字符串函数进行数据处理,以下是几个示例:1. 使用CONCAT函数连接字符串路径:假设我们要从以下JSON对象中提取州名称:
{ "name": "John", "age": 30, "address": { "street": "123 Main St", "city": "Anytown", "state": "TX", "zip": "12345" } }
我们可以使用以下命令从中提取州名称:
SELECT JSON_EXTRACT('{ "name": "John", "age": 30, "address": { "street": "123 Main St", "city": "Anytown", "state": "TX", "zip": "12345" } }', CONCAT('$.address.', 'state')); -- 返回值为"TX"
2. 使用SUBSTRING函数截取路径:假设我们有以下JSON对象:
{ "product_name": "Awesome Product", "features": [ { "name": "Feature A", "description": "This is feature A." }, { "name": "Feature B", "description": "This is feature B." }, { "name": "Feature C", "description": "This is feature C." } ] }
我们可以使用以下命令只获取features子节点:
SELECT JSON_EXTRACT('{ "product_name": "Awesome Product", "features": [ { "name": "Feature A", "description": "This is feature A." }, { "name": "Feature B", "description": "This is feature B." }, { "name": "Feature C", "description": "This is feature C." } ] }', SUBSTRING('$.features', 2)); -- 返回值为[{...}]