SQL Language Guide > SQL Language Guide > Working with JSON Data > JSON Concepts > JSON Predicates
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JSON Predicates
JSON predicates work against a sequence as follows:
• In lax mode, arrays are unwrapped.
• The predicate is evaluated for all values of the input sequence.
• The return sequence is the set of values for which the predicate is true.
• If the input sequence contains three items, but only two pass the filter qualifications, then the output sequence will contain those two values.
A predicate can return true, false, or a JSON unknown (a predicate error).
An example of a predicate returning unknown: (@.hours > 9) if @.hours is a string scalar.
The following JSON predicates are available:
• EXISTS
• Comparison operators
• SIMILAR TO
• STARTS WITH
• IS UNKNOWN
EXISTS
The EXISTS predicate tests for the existence of a key. Specifically, it tests whether a member accessor returns at least one JSON value. It can be used to probe for a member before accessing it. EXISTS is especially useful for preventing errors in STRICT mode.
Example:
STRICT $ ? (EXISTS (@.name)).name
This JSON path returns an empty sequence if the base object does not contain the key “name”.
Comparison Operators
The following comparison operators are supported:
Comparison Operator
Description
==
Equal to
!= or <>
Not equal to
<
Less than
>
Greater than
<=
Less than or equal to
>=
Greater than or equal to
SIMILAR TO
The SIMILAR TO predicate tests if a string matches a pattern. The syntax is identical to SQL’s SIMILAR TO except it supports only _ and %. In the JSON standard, the standard escape character is the backslash (\); therefore, unlike SQL’s SIMILAR TO, the escape “\\” clause is assumed and cannot be specified or changed.
Example:
SELECT c FROM t01 WHERE JSON_EXISTS (c, 'LAX $ ? (@.col.Name SIMILAR TO "d%")
STARTS WITH
The STARTS WITH predicate tests for an initial substring.
A STARTS WITH B returns true if string A starts with string B (that is, if string B is a prefix of string A).
No wildcards are allowed in either operand. To use wildcards, the SIMILAR TO predicate must be used.
If the second operand of STARTS WITH is a sequence, then TRUE is returned if the first operand starts with any value within the second operand’s sequence.
IS UNKNOWN
The IS UNKNOWN predicate tests for JSON unknown results. It lets you find invalid rows by testing a result to see if it returned a JSON unknown (which is typically an error).
Example:
SELECT id
FROM employees
WHERE JSON_EXISTS (datacol,
'LAX $ ? ( (@.age > 18) IS UNKNOWN')
This query returns all rows where (@.age > 18) returns an JSON unknown rather than true or false. In this case, most likely @.age is a non-numeric value.
Last modified date: 11/09/2022