---
layout: default
title: Complex Queries
parent: SQL
grand_parent: SQL and PPL
nav_order: 6
---

# Complex queries

Besides simple SFW (`SELECT-FROM-WHERE`) queries, the SQL plugin supports complex queries such as subquery, join, union, and minus. These queries operate on more than one OpenSearch index. To examine how these queries execute behind the scenes, use the `explain` operation.


## Joins

OpenSearch SQL supports inner joins, cross joins, and left outer joins.

### Constraints

Joins have a number of constraints:

1. You can only join two indices.
1. You must use aliases for indices (e.g. `people p`).
1. Within an ON clause, you can only use AND conditions.
1. In a WHERE statement, don't combine trees that contain multiple indices. For example, the following statement works:

   ```
   WHERE (a.type1 > 3 OR a.type1 < 0) AND (b.type2 > 4 OR b.type2 < -1)
   ```

   The following statement does not:

   ```
   WHERE (a.type1 > 3 OR b.type2 < 0) AND (a.type1 > 4 OR b.type2 < -1)
   ```

1. You can't use GROUP BY or ORDER BY for results.
1. LIMIT with OFFSET (e.g. `LIMIT 25 OFFSET 25`) is not supported.

### Description

The `JOIN` clause combines columns from one or more indices using values common to each.

### Syntax

Rule `tableSource`:

![tableSource]({{site.url}}{{site.baseurl}}/images/tableSource.png)

Rule `joinPart`:

![joinPart]({{site.url}}{{site.baseurl}}/images/joinPart.png)

### Example 1: Inner join

Inner join creates a new result set by combining columns of two indices based on your join predicates. It iterates the two indices and compares each document to find the ones that satisfy the join predicates. You can optionally precede the `JOIN` clause with an `INNER` keyword.

The join predicate(s) is specified by the ON clause.

SQL query:

```sql
SELECT
  a.account_number, a.firstname, a.lastname,
  e.id, e.name
FROM accounts a
JOIN employees_nested e
 ON a.account_number = e.id
```

Explain:

The `explain` output is complicated, because a `JOIN` clause is associated with two OpenSearch DSL queries that execute in separate query planner frameworks. You can interpret it by examining the `Physical Plan` and `Logical Plan` objects.

```json
{
  "Physical Plan" : {
    "Project [ columns=[a.account_number, a.firstname, a.lastname, e.name, e.id] ]" : {
      "Top [ count=200 ]" : {
        "BlockHashJoin[ conditions=( a.account_number = e.id ), type=JOIN, blockSize=[FixedBlockSize with size=10000] ]" : {
          "Scroll [ employees_nested as e, pageSize=10000 ]" : {
            "request" : {
              "size" : 200,
              "from" : 0,
              "_source" : {
                "excludes" : [ ],
                "includes" : [
                  "id",
                  "name"
                ]
              }
            }
          },
          "Scroll [ accounts as a, pageSize=10000 ]" : {
            "request" : {
              "size" : 200,
              "from" : 0,
              "_source" : {
                "excludes" : [ ],
                "includes" : [
                  "account_number",
                  "firstname",
                  "lastname"
                ]
              }
            }
          },
          "useTermsFilterOptimization" : false
        }
      }
    }
  },
  "description" : "Hash Join algorithm builds hash table based on result of first query, and then probes hash table to find matched rows for each row returned by second query",
  "Logical Plan" : {
    "Project [ columns=[a.account_number, a.firstname, a.lastname, e.name, e.id] ]" : {
      "Top [ count=200 ]" : {
        "Join [ conditions=( a.account_number = e.id ) type=JOIN ]" : {
          "Group" : [
            {
              "Project [ columns=[a.account_number, a.firstname, a.lastname] ]" : {
                "TableScan" : {
                  "tableAlias" : "a",
                  "tableName" : "accounts"
                }
              }
            },
            {
              "Project [ columns=[e.name, e.id] ]" : {
                "TableScan" : {
                  "tableAlias" : "e",
                  "tableName" : "employees_nested"
                }
              }
            }
          ]
        }
      }
    }
  }
}
```

Result set:

| a.account_number | a.firstname | a.lastname | e.id | e.name
:--- | :--- | :--- | :--- | :---
6 | Hattie | Bond | 6 | Jane Smith

### Example 2: Cross join

Cross join, also known as cartesian join, combines each document from the first index with each document from the second.
The result set is the the cartesian product of documents of both indices.
This operation is similar to the inner join without the `ON` clause that specifies the join condition.

It's risky to perform cross join on two indices of large or even medium size. It might trigger a circuit breaker that terminates the query to avoid running out of memory.
{: .warning }

SQL query:

```sql
SELECT
  a.account_number, a.firstname, a.lastname,
  e.id, e.name
FROM accounts a
JOIN employees_nested e
```

Result set:

| a.account_number | a.firstname | a.lastname | e.id | e.name
:--- | :--- | :--- | :--- | :---
1 | Amber | Duke | 3 | Bob Smith
1 | Amber | Duke | 4 | Susan Smith
1 | Amber | Duke | 6 | Jane Smith
6 | Hattie | Bond | 3 | Bob Smith
6 | Hattie | Bond | 4 | Susan Smith
6 | Hattie | Bond | 6 | Jane Smith
13 | Nanette | Bates | 3 | Bob Smith
13 | Nanette | Bates | 4 | Susan Smith
13 | Nanette | Bates | 6 | Jane Smith
18 | Dale | Adams | 3 | Bob Smith
18 | Dale | Adams | 4 | Susan Smith
18 | Dale | Adams | 6 | Jane Smith

### Example 3: Left outer join

Use left outer join to retain rows from the first index if it does not satisfy the join predicate. The keyword `OUTER` is optional.

SQL query:

```sql
SELECT
  a.account_number, a.firstname, a.lastname,
  e.id, e.name
FROM accounts a
LEFT JOIN employees_nested e
 ON a.account_number = e.id
```

Result set:

| a.account_number | a.firstname | a.lastname | e.id | e.name
:--- | :--- | :--- | :--- | :---
1 | Amber | Duke | null | null
6 | Hattie | Bond | 6 | Jane Smith
13 | Nanette | Bates | null | null
18 | Dale | Adams | null | null

## Subquery

A subquery is a complete `SELECT` statement used within another statement and enclosed in parenthesis.
From the explain output, you can see that some subqueries are actually transformed to an equivalent join query to execute.

### Example 1: Table subquery

SQL query:

```sql
SELECT a1.firstname, a1.lastname, a1.balance
FROM accounts a1
WHERE a1.account_number IN (
  SELECT a2.account_number
  FROM accounts a2
  WHERE a2.balance > 10000
)
```

Explain:

```json
{
  "Physical Plan" : {
    "Project [ columns=[a1.balance, a1.firstname, a1.lastname] ]" : {
      "Top [ count=200 ]" : {
        "BlockHashJoin[ conditions=( a1.account_number = a2.account_number ), type=JOIN, blockSize=[FixedBlockSize with size=10000] ]" : {
          "Scroll [ accounts as a2, pageSize=10000 ]" : {
            "request" : {
              "size" : 200,
              "query" : {
                "bool" : {
                  "filter" : [
                    {
                      "bool" : {
                        "adjust_pure_negative" : true,
                        "must" : [
                          {
                            "bool" : {
                              "adjust_pure_negative" : true,
                              "must" : [
                                {
                                  "bool" : {
                                    "adjust_pure_negative" : true,
                                    "must_not" : [
                                      {
                                        "bool" : {
                                          "adjust_pure_negative" : true,
                                          "must_not" : [
                                            {
                                              "exists" : {
                                                "field" : "account_number",
                                                "boost" : 1
                                              }
                                            }
                                          ],
                                          "boost" : 1
                                        }
                                      }
                                    ],
                                    "boost" : 1
                                  }
                                },
                                {
                                  "range" : {
                                    "balance" : {
                                      "include_lower" : false,
                                      "include_upper" : true,
                                      "from" : 10000,
                                      "boost" : 1,
                                      "to" : null
                                    }
                                  }
                                }
                              ],
                              "boost" : 1
                            }
                          }
                        ],
                        "boost" : 1
                      }
                    }
                  ],
                  "adjust_pure_negative" : true,
                  "boost" : 1
                }
              },
              "from" : 0
            }
          },
          "Scroll [ accounts as a1, pageSize=10000 ]" : {
            "request" : {
              "size" : 200,
              "from" : 0,
              "_source" : {
                "excludes" : [ ],
                "includes" : [
                  "firstname",
                  "lastname",
                  "balance",
                  "account_number"
                ]
              }
            }
          },
          "useTermsFilterOptimization" : false
        }
      }
    }
  },
  "description" : "Hash Join algorithm builds hash table based on result of first query, and then probes hash table to find matched rows for each row returned by second query",
  "Logical Plan" : {
    "Project [ columns=[a1.balance, a1.firstname, a1.lastname] ]" : {
      "Top [ count=200 ]" : {
        "Join [ conditions=( a1.account_number = a2.account_number ) type=JOIN ]" : {
          "Group" : [
            {
              "Project [ columns=[a1.balance, a1.firstname, a1.lastname, a1.account_number] ]" : {
                "TableScan" : {
                  "tableAlias" : "a1",
                  "tableName" : "accounts"
                }
              }
            },
            {
              "Project [ columns=[a2.account_number] ]" : {
                "Filter [ conditions=[AND ( AND account_number ISN null, AND balance GT 10000 ) ] ]" : {
                  "TableScan" : {
                    "tableAlias" : "a2",
                    "tableName" : "accounts"
                  }
                }
              }
            }
          ]
        }
      }
    }
  }
}
```

Result set:

| a1.firstname | a1.lastname | a1.balance
:--- | :--- | :---
Amber | Duke | 39225
Nanette | Bates | 32838

### Example 2: From subquery

SQL query:

```sql
SELECT a.f, a.l, a.a
FROM (
  SELECT firstname AS f, lastname AS l, age AS a
  FROM accounts
  WHERE age > 30
) AS a
```

Explain:

```json
{
  "from" : 0,
  "size" : 200,
  "query" : {
    "bool" : {
      "filter" : [
        {
          "bool" : {
            "must" : [
              {
                "range" : {
                  "age" : {
                    "from" : 30,
                    "to" : null,
                    "include_lower" : false,
                    "include_upper" : true,
                    "boost" : 1.0
                  }
                }
              }
            ],
            "adjust_pure_negative" : true,
            "boost" : 1.0
          }
        }
      ],
      "adjust_pure_negative" : true,
      "boost" : 1.0
    }
  },
  "_source" : {
    "includes" : [
      "firstname",
      "lastname",
      "age"
    ],
    "excludes" : [ ]
  }
}
```

Result set:

| f | l | a
:--- | :--- | :---
Amber | Duke | 32
Dale | Adams | 33
Hattie | Bond | 36
