Added a page about how to use DQL

_dashboards/dql.md

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+---
+layout: default
+title: Dashboards Query Language
+nav_order: 99
+---
+
+# Dashboards Query Language
+
+Similar to the [Query DSL]({{site.url}}{{site.baseurl}}/opensearch/query-dsl/index) that lets you use the HTTP request body to search for data, you can use the Dashbaords Query Language (DQL) in OpenSearch Dashboards to search for data and visualizations.
+
+For example, if you want to see all visualizations that relate to Windows 8, enter `win 8` into the search field, and Dashboards refreshes to display all data related to Windows 8.
+
+Just like query DSL, DQL comes in a handful of varieties, so use whichever best fits your use case.
+
+---
+
+#### Table of contents
+1. TOC
+{:toc}
+
+---
+
+## Terms query
+
+The most basic query is to just specify the term you're searching for.
+
+```
+machine.os.keyword:win 8
+```
+
+DQL also supports wildcards, so you can search for any terms that match your pattern.
+
+```
+machine.os.keyword:win*
+```
+
+To check if a field exists or has any data, use a wildcard to see if Dashboards returns any results.
+
+```
+machine.os.keyword:*
+```
+
+## Boolean query
+
+To mix and match, or even combine, multiple queries for more refined results, you can use the boolean operators `and`, `or`, and `not`. DQL is not case sensitive, so `AND` and `and` are the same.
+
+```
+machine.os.keyword:win 8 and response.keyword:200
+```
+
+The following example demonstrates how to use multiple operators in one query.
+
+```
+machine.os.keyword:win 8 or response.keyword:200 and host.keyword:www.example.com
+```
+
+Remember that logical precedence for boolean operators follows the order `not`, `and`, and `or`, so if you have an expression like the previous example, `response.keyword:200 and host.keyword:www.example.com` gets evaluated first, and then Dashboards uses that result to compare with `machine.os.keyword:win 8`.
+
+To avoid confusion, we recommend using parentheses to dictate the order you want to evaluate in. If you want to evaluate `machine.os.keyword:win 8 or response.keyword:200` first, your expression becomes:
+
+```
+(machine.os.keyword:win 8 or response.keyword:200) and host.keyword:www.example.com
+```
+
+## Date and range queries
+
+DQL also supports inequalities if you're using numeric inequalities.
+
+```
+bytes >= 15 and memory < 15
+```
+
+Similarly, you can use the same method to find a date before or after your query. `>` indicates a search for a date after your specified date, and `<` returns dates before.
+
+```
+@timestamp > "2020-12-14T09:35:33"
+```
+
+## Nested field query
+
+If you have a document with nested fields, you have to specify which parts of the document you want to retrieve.
+
+Suppose that you have the following document:
+
+```json
+{
+  "superheroes":[
+    {
+      "hero-name": "Superman",
+      "real-identity": "Clark Kent",
+      "age": 28
+    },
+    {
+      "hero-name": "Batman",
+      "real-identity": "Bruce Wayne",
+      "age": 26
+    },
+    {
+      "hero-name": "Flash",
+      "real-identity": "Barry Allen",
+      "age": 28
+    },
+    {
+      "hero-name": "Robin",
+      "real-identity": "Dick Grayson",
+      "age": 15
+    }
+  ]
+}
+```
+
+The following example demonstrates how to use DQL to retrieve a specific field.
+
+```
+superheroes: {hero-name: Superman}
+```
+
+If you want to retrieve multiple objects from your document, just specify all of the fields you want to retrieve.
+
+```
+superheroes: {hero-name: Superman} and superheroes: {hero-name: Batman}
+```
+
+The previous boolean and range queries still work, so you can submit a more refined query.
+
+```
+superheroes: {hero-name: Superman and age < 50}
+```
+
+If your document has an object nested within another object, you can still retrieve data by specifying all of the levels.
+
+```
+justice-league.superheroes: {hero-name:Superman}
+```