JSON Expressions

Sample data

{
  "name": {"first": "Tom", "last": "Anderson"},
  "age":37,
  "children": ["Sara","Alex","Jack"],
  "fav.movie": "Deer Hunter",
  "friends": [
    {"first": "Dale", "last": "Murphy", "age": 44},
    {"first": "Roger", "last": "Craig", "age": 68},
    {"first": "Jane", "last": "Murphy", "age": 47}
  ],
    "followers": {
        "Group1": [
            {"first": "John", "last": "Shavor", "age": 22},
            {"first": "Ken", "last": "Miller", "age": 33}
        ],
        "Group2": [
            {"first": "Alice", "last": "Murphy", "age": 33},
            {"first": "Brian", "last": "Craig", "age": 44}
        ]
    }
   "ops": {
       "functionA": {"numArgs": 2},
    "functionB": {"numArgs": 3},
    "functionC": {"variadic": true}
  }
}

Basic expressions

Identifier

Source Dereference (.) The source dereference operator can be used to specify columns by dereferencing the source stream or table. The -> dereference selects a key in a nested JSON object.

SELECT demo.age FROM demo
{"age" : 37}

SELECT demo.name->first FROM demo
{"first" : "Tom"}

SELECT name->first AS fname FROM demo
{"fname": "Tom"}

Index expression

Index Expressions allow you to select a specific element in a list. It should look similar to array access in common programming languages.The index value starts with 0, -1 is the starting position from the end, and so on.

SELECT children FROM demo
{
    "children": ["Sara","Alex","Jack"]
}

SELECT children[0] FROM demo
{
    "children": "Sara"
}
SELECT children[1] FROM demo
{
    "children": "Alex"
}
SELECT children[-1] FROM demo
{
    "children": "Jack"
}
SELECT children[-2] FROM demo
{
    "children": "Alex"
}
SELECT d.friends[0]->last FROM demo AS d
{
    "last" : "Murphy"
}

Slicing

Slices allow you to select a contiguous subset of an array.

field[from:to)is the interval before closing and opening, excluding to. If from is not specified, then it means start from the 1st element of array; If to is not specified, then it means end with the last element of array.

SELECT children[0:1] FROM demo
{
    "children": ["Sara"]
}
SELECT children[1:-1] FROM demo
{
    "children": ["Alex"]
}
SELECT children[0:-1] FROM demo
{
    "children": ["Sara","Alex"]
}

SELECT children[:] FROM demo == SELECT children FROM demo
{
    "children": ["Sara","Alex","Jack"]
}

SELECT children[:2] FROM demo
{
    "children": ["Sara","Alex"]
}

SELECT followers->Group1[:1]->first FROM demo
{
    "first": ["John"]
}

Json Path functions

eKuiper provides a list of functions to allow to execute json path over struct or array columns or values. The functions are:

json_path_exists(col, jsonpath)
json_path_query(col, jsonpath)
json_path_query_first(col, jsonpath)

Please refer to json functions for detail.

All these functions share the same parameter signatures, among which, the second parameter is a jsonpath string. The jsonpath grammer used by eKuiper is based on JsonPath.

The basic grammar of those expressions is to use the keys part of the JSON objects combined with some elements:

• Dots . to move into a tree
• Brackets [] for access to a given array member coupled with a position. It can also access to a map field.
• Variables, with $ representing a JSON text and @ for result path evaluations.

So for example, when applied to the previous JSON data sample we can reach the following parts of the tree with these expressions:

• $.age refers to 37.
• $.friends.first refers to “dale”.
• $.friends refers to the full array of friends.
• $.friends[0] refers to the first friend listed in the previous array (contrary to arrays members are zero-based).
• $.friends[0][lastname] refers to the lastname of the first friend listed. Use bracket if there are reserved words or special characters (such as space ‘ ‘, ‘.’ and Chinese etc) in the field key.
• $.friends[? @.age>60].first or $.friends[? (@.age>60)].first refers to the first name of the friends whose age is bigger than 60. Notice that the space between ? and the condition is required even the condition is with braces.

Developers can use the json functions in the SQL statement. Here are some examples.

• Select the lastname of group1 followers ```sql SELECT json_path_query(followers, “$.Group1[*].last”) FROM demo

[“Shavor”,”Miller”]

- Select the lastname if any of the group1 followers is older than 60
```sql
SELECT name->last FROM demo where json_path_exists(followers, "$.Group1[? @.age>30]")
"Anderson"

• Select the follower’s lastname from group1 whose age is bigger than 30 ```sql SELECT json_path_exists(followers, “$.Group1[? @.age>30].last”) FROM demo

[“Miller”]

- Assume there is a field in follows with reserved words or chars like dot `my.follower`, use bracket to access it.
```sql
SELECT json_path_exists(followers, "$[\"my.follower\"]") FROM demo
["Miller"]

Projections - NOT SUPPORT YET

List & Slice projections

A wildcard expression creates a list projection, which is a projection over a JSON array.

SELECT demo.friends[*]->first FROM demo
{
    "first": ["Dale", "Roger", "Jane"]
}

SELECT friends[:1]->first FROM demo
{
    "first": ["Dale", "Roger"]
}

Object projections

SELECT ops->*->numArgs FROM demo
{ "numArgs" : [2, 3] }