So I made this system to store custom objects with custom fields for an app that I'm developing. First I have object_def where I save the object definitions:
id | name | fields
------------------------------------------------------------
101 | Group 1 | [{"name": "Title", "id": "AbCdE123"}, ...]
102 | Group 2 | [{"name": "Name", "id": "FgHiJ456"}, ...]
So we have ID (INT), name (VARCHAR) and fields (LONGTEXT). In fields are the object fields like this: {id: string, type: string, name: string}[].
Now In the object table, I have this:
id | object_def_id | object_values
------------------------------------------------------------
235 | 101 | {"AbCdE123": "The Object", ... }
236 | 102 | {"FgHiJ456": "John Perez", ... }
Where object_values is a LONGTEXT also. With that system, I'm able to show the objects on a table in my app using JSON.parse().
Now I've learned that there is a JSON type in MySQL and I want it to use it to do queries and stuff (I'm really new to this).
I've changed the LONGTEXT to JSON and now I wanted to do a SELECT that show the results like this:
#Select objects in group 1:
id | group | Title | ... | other_custom_field
-------------------------------------------------------
235 | Group 1 | The Object | ... | other_custom_value
#Select objects in group 2:
id | group | Name | ... | other_custom_field
-------------------------------------------------------
236 | Group 2 | John Perez | ... | other_custom_value
Id, then group name (I can do this with INNER JOIN) and then all the custom fields with the respective values.
Is this possible? How can I achieve this (hopefully without changing my database structure)? I'm learning MySQL, SQL and databases as I go so I really appreciate your help. Thanks!
Problems I see with your design:
Incorrect JSON format.
[{name: 'Title', id: 'AbCdE123'}, ...]
Should be:
[{"name": "Title", "id": "AbCdE123"}, ...]
You should use the JSON data type instead of LONGTEXT, because JSON will at least reject invalid JSON syntax.
Setting column headings based on data. You can't do this in SQL. Columns and headings must be fixed at the time you prepare the query. You can't do an SQL query that changes its own column headings.
Your object def has an array of attributes, but there's no way in MySQL 5.7 to loop over the "rows" of a JSON array. You'll need to use the JSON_TABLE() in MySQL 8.0.
That will get you closer to being able to look up object values, but then you'll still have to pivot the data into the result set you describe, with one attribute in each column, as if the data had been stored in a traditional way. But SQL doesn't allow you to do dynamic pivoting in a single query. You can't make an SQL query that dynamically grows its own select-list based on the data it finds.
This all makes me wonder...
Why don't you just store the data in the traditional way?
Create a table per object type. Add one column to that table per attribute. That way you get column names. You get column types. You get column constraints — for example, how would you simulate NOT NULL or UNIQUE in your current system?
If you don't want to use SQL, then don't. There are alternatives, like document databases or key/value databases. But don't torture poor SQL by using it to implement an Inner-Platform.
Related
I have a table which's name is users in my MySQL database, and I am using this DB with Ruby on Rails application with ORM structure for years. The table has id field and this field is configured as AI (auto-increment), BIGINT.
Example of my users table;
+----+---------+
| id | name |
+----+---------+
| 1 | John |
| 2 | Tommy |
| 3 | ... |
| 4 | ... |
| 5 | ... |
| 6 | ... |
+----+---------+
The problem I am facing is when I execute the following query I get unexpected rows.
SELECT * FROM users WHERE id = '1AW3F4SEFR';
This query is returning the exact same value with the following query,
SELECT * FROM users WHERE id = 1;
I do not know why SQL let me use strings in WHERE clause on a data type INT. And as we can see from the example, my DB converts the strings I gave to the integer at position 0. I mean, I search for 1AW3F4SEFR and I expect not to get any result. But SQL statement returns the results for id = 1.
In Oracle SQL, the behavior of this exact same query is completely different. So, I believe there is something different on MySQL. But I am not sure about what causes this.
As has been explained in the request comments, MySQL has a weird way of converting strings to numbers. It simply takes as much of a string from the left as is numeric and ignores the rest. If the string doesn't start with a number the conversion defaults to 0.
Examples: '123' => 123, '12.3' => 12.3, '.123' => 0.123, '12A3' => 12, 'A123' => 0, '.1A1.' => 0.1
Demo: https://dbfiddle.uk/?rdbms=mysql_8.0&fiddle=55cd18865fad4738d03bf28082217ca8
That MySQL doesn't raise an error here as other DBMS do, can easily lead to undesired query results that get a long time undetected.
The solution is easy though: Don't let this happen. Don't compare a numeric column with a string. If the ID '1AW3F4SEFR' is entered in some app, raise an error in the app or even prevent this value from being entered. When running the SQL query, make sure to pass a numeric value, so '1AW3F4SEFR' cannot even make it into the DBMS. (Look up how to use prepared statements and pass parameters of different types to the database system in your programming language.)
If for some reason you want to pass a string for the ID instead (I cannot think of any such reason though) and want to make your query fail-safe by not returning any row in case of an ID like '1AW3F4SEFR', check whether the ID string represents an integer value in the query. You can use REGEXP for this.
SELECT * FROM users WHERE id = #id AND #id REGEXP '^[0-9]+$';
Thus you only consider integer ID strings and still enable the DBMS to use an index when looking up the ID.
Demo: https://dbfiddle.uk/?rdbms=mysql_8.0&fiddle=56f8ee902342752933c20b8762f14dbb
I have a table with JSON data like this:
{"a": [{"color": "blue", "value": 15}, {"color": "red", "value": 30}]}
I need to get the "value" that is inside the same object of "blue".
I thought to use the code below:
SELECT JSON_EXTRACT(my_data, '$.a[0].value');
The problem is that the "blue" object can be in any index of the array.
So, is there a way to retrieve the index first and then i'll query using the right index?
UPDATE
The Barmar's answer works but it needs to wrap in JSON_UNQUOTE()
Use JSON_SEARCH() to find the path to blue.
SELECT JSON_EXTRACT(my_data, JSON_UNQUOTE(REPLACE(JSON_SEARCH(my_data, 'one', 'blue'), '.color', '.value')))
JSON_SEARCH will return a string like $.a[0].color. REPLACE changes that to $.a[0].value, then you extract that element.
DEMO
Here's an example of using JSON_TABLE():
select j.* from d, json_table(d.data, '$.a[*]' columns (
color varchar(20) path '$.color',
value int path '$.value')
) as j;
+-------+-------+
| color | value |
+-------+-------+
| blue | 15 |
| red | 30 |
+-------+-------+
You can then apply conditions in the WHERE clause, as if you had stored the data in a normal table.
select j.* from d, json_table(d.data, '$.a[*]' columns (
color varchar(20) path '$.color',
value int path '$.value')
) as j
where j.color = 'blue';
+-------+-------+
| color | value |
+-------+-------+
| blue | 15 |
+-------+-------+
This requires you to write a complex query like this EVERY TIME you query the JSON data.
One wonders if it would have been easier to store the JSON in a normal table from the start.
I often recommend to MySQL users that storing data as JSON makes more work for you, if you need to make SQL expressions to reference individual fields within the JSON. I wouldn't use JSON in these cases, I'd explode the JSON array into rows, and the JSON fields into columns of a set of normal tables. Then you can write simpler queries, you can optimize with indexes, and you can use constraints and data types properly.
JSON is the most easily misused feature of the recent MySQL releases.
Consider the following table
+-------------+--------------------------------------------------------------------------+
|company_name | products |
+-------------+--------------------------------------------------------------------------+
| comp1 | [{"name": "prod1","pending": false}, {"name": "prod2","pending": true}] |
+-------------+--------------------------------------------------------------------------+
Suppose I want to retrieve results based on the the value of "name" field inside "products" column, for ex: if "pending" = "true", then display the company_name and also the product name of the particular JSON object inside the List or Array.
Result should look something like,
+-------------+-------------------------------+
|company_name | product_name |
+-------------+-------------------------------+
| comp1 | prod2 |
+-------------+-------------------------------+
You should use a program in python, perl etc that retrieve json and convert it to array and you print the column of array that you need.
use index to get value $[ index ]
Code will be like this:
Select company_name,json_extract(products,'$[1].name') product_name from table where json_extract(products,'$[1].pending')='false' ;
Note: create a dummy table with numbers and join it for index
Suppose I have the following MySQL table structure:
==========================================================
| id | name | address |
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
| 1 | Dawn | {"state": "Maryland", "city": "Annapolis"} |
| 2 | Pablo| {"state": "Ohio", "city": "Columbus"} |
The address column is of json type. I use the bookshelf and the knex ORM in my application, I need to do query so that it will fetch name and the state name without city.
My plain SQL query will look like this:
select name, address->"$.state"
from user
OR
select name, json_extract(address, "$.state")
from user
But when I do it using Bookshelf:
user.select('name', 'address->"$.state"').fetch().then(....)
I'm getting an invalid error in SQL command due to the quote and backtick format.
The SQL generated using bookshelf is as below:
select 'address->\" '.'state\" ', 'name' from `user.
Any suggestions to avoid the backtick mark issue around Dot(.) character ?? Or some other way of approach??
PS: This just an example
You can generate those queries like this:
// select name, address->"$.state" from user
knex('user').select('name', knex.raw('??->"$.state"', ['address']))
// select name, json_extract(address, "$.state") from user
knex('user').select('name', knex.raw('json_extract(??, "$.state")', ['address']))
I have an SQL database field that contains JSON type data stored.
-----------------------------
id | tags |
-----------------------------
1 | ['cat','dog'] |
2 | ['lion','cat','dog'] |
I want to select from this table by passing where condition as cat and get all the JSON fields. How would I do this?
Use the JSON_EXTRACT function as of MySQL 5.7.8. Extract from
https://dev.mysql.com/doc/refman/5.7/en/json.html#json-paths
A JSON path expression selects a value within a JSON document.
Path expressions are useful with functions that extract parts of or modify a JSON document, to specify where within that document to operate. For example, the following query extracts from a JSON document the value of the member with the name key:
mysql> SELECT JSON_EXTRACT('{"id": 14, "name": "Aztalan"}', '$.name');
+---------------------------------------------------------+
| JSON_EXTRACT('{"id": 14, "name": "Aztalan"}', '$.name') |
+---------------------------------------------------------+
| "Aztalan" |
+---------------------------------------------------------+
Note in the example they are inline creatinng a JSON object so for you
:'{"id": 14, "name": "Aztalan"}' would actually be your column value.
JSON manipulation in MySQL is slow. I suggest that you use REGEXP.
// get all rows having "cat" value
select * from animals_tbl
where tags regexp 'cat';
// get all rows based on multiple filters
select * from animals_tbl
where tags regexp 'lion|cat';