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## 15.2. Match

Unlike SQL which operates on sets, Cypher predominantly works on sub-graphs. The relational equivalent is the current set of tuples being evaluated during a `SELECT` query.

The shape of the sub-graph is specified in the `MATCH` clause. The `MATCH` clause is analogous to the `JOIN` in SQL. A normal a→b relationship is an inner join between nodes a and b — both sides have to have at least one match, or nothing is returned.

We’ll start with a simple example, where we find all email addresses that are connected to the person “Anakin”. This is an ordinary one-to-many relationship.

SQL Query.

    SELECT "Email".*
    FROM "Person"
    JOIN "Email" ON "Person".id = "Email".person_id
    WHERE "Person".name = 'Anakin'

ADDRESS | COMMENT | PERSON_ID
--- | --- | ---
2 rows
`anakin@example.com` | `home` | `1`
`anakin@example.org` | `work` | `1`

Cypher Query.

    START person=node:Person(name = 'Anakin')
    MATCH person-[:email]->email
    RETURN email

email
2 rows
`Node[6]{address:"anakin@example.com",comment:"home"}`
`Node[7]{address:"anakin@example.org",comment:"work"}`

There is no join table here, but if one is necessary the next example will show how to do that, writing the pattern relationship like so: `-[r:belongs_to]->` will introduce (the equivalent of) join table available as the variable `r`. In reality this is a named relationship in Cypher, so we’re saying “join `Person` to `Group` via `belongs_to`.” To illustrate this, consider this image, comparing the SQL model and Neo4j/Cypher.

And here are example queries:

SQL Query.

    SELECT "Group".*, "Person_Group".*
    FROM "Person"
    JOIN "Person_Group" ON "Person".id = "Person_Group".person_id
    JOIN "Group" ON "Person_Group".Group_id="Group".id
    WHERE "Person".name = 'Bridget'

NAME | ID | BELONGS_TO_GROUP_ID | PERSON_ID | GROUP_ID
--- | --- | --- | --- | ---
1 rows
`Admin` | `4` | `3` | `2` | `4`

Cypher Query.

    START person=node:Person(name = 'Bridget')
    MATCH person-[r:belongs_to]->group
    RETURN group, r

group | r
--- | ---
1 row
`Node[5]{name:"Admin",id:4}` | `:belongs_to[0]{}`

An outer join is just as easy. Add `OPTIONAL` before the match and it’s an optional relationship between nodes — the outer join of Cypher.

Whether it’s a left outer join, or a right outer join is defined by which side of the pattern has a starting point. This example is a left outer join, because the bound node is on the left side:

SQL Query.

    SELECT "Person".name, "Email".address
    FROM "Person" LEFT
    JOIN "Email" ON "Person".id = "Email".person_id

NAME | ADDRESS
--- | ---
3 rows
`Anakin` | `anakin@example.com`
`Anakin` | `anakin@example.org`
`Bridget` | `<null>`

Cypher Query.

    START person=node:Person('name: *')
    OPTIONAL MATCH person-[:email]->email
    RETURN person.name, email.address

person.name | email.address
--- | ---
3 rows
`"Anakin"` | `"anakin@example.com"`
`"Anakin"` | `"anakin@example.org"`
`"Bridget"` | `<null>`

Relationships in Neo4j are first class citizens — it’s like the SQL tables are pre-joined with each other. So, naturally, Cypher is designed to be able to handle highly connected data easily.

One such domain is tree structures — anyone that has tried storing tree structures in SQL knows that you have to work hard to get around the limitations of the relational model. There are even books on the subject.

To find all the groups and sub-groups that Bridget belongs to, this query is enough in Cypher:

Cypher Query.

    START person=node:Person('name: Bridget')
    MATCH person-[:belongs_to*]->group
    RETURN person.name, group.name

person.name | group.name
--- | ---
3 rows
`"Bridget"` | `"Admin"`
`"Bridget"` | `"Technichian"`
`"Bridget"` | `"User"`

The * after the relationship type means that there can be multiple hops across `belongs_to` relationships between group and user. Some SQL dialects have recursive abilities, that allow the expression of queries like this, but you may have a hard time wrapping your head around those. Expressing something like this in SQL is hugely impractical if not practically impossible.

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