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• Query pattern anatomy

In this section we give an overview of patterns - building blocks of majority of Graql queries.

Query pattern anatomy

As we have seen before, at the core of each query sits a query pattern that describes a subgraph of our particular interest. Here we examine the structure of query patterns closer. In general, patterns can be thought of as different arrangements of statement collections. Graql statements constitute the smallest building blocks of queries. Let’s have a close look at the constructs of a basic match clause.

• Each statement starts with a variable (V) providing a concept reference. We can reference both data and schema concepts via variables. A Graql variable is prefixed with a dollar sign $.

• The variable is followed by a comma-separated list of properties (P1, P2, P3) describing the concepts the variable refers to. Here we can see that all the concepts that variable $p refers to, must be of type person. The matched instances are expected to own an attribute of type name with the value of "Bob". Additionally, we require the concepts to own an attribute of type phone-number with any value. We signal that we want to fetch the owned phone-numbers as well by defining an extra $phone variable. Consequently, after performing a match on this statement, we should obtain pairs of concepts that satisfy our statement.

• We mark the end of the statement with a semi-colon ;.

There is some freedom in forming and composing our statements. For example, as shown below, we could write our single statement with three properties as three combined statements.

$p isa person;
$p has name 'Bob';
$p has phone-number $phone;

Consequently, we arrive at the subject of pattern composition. We already know that statements are the smallest building blocks, however, we have a number of possibilities for arranging them together. By doing so, we can express more complex pattern scenarios and their corresponding subgraphs. We allow the following ways to arrange statements together.

1. Statement: simplest possible arrangement - a single basic building block as explained above.
2. Conjunction: a set of patterns where to satisfy a match, all patterns must be matched. We form conjunctions by separating the partaking patterns with semi-colons ;.
3. Disjunction: a set of patterns where to satisfy a match, at least one pattern must be matched. We form disjunctions by enclosing the partaking patterns within curly braces and interleaving them with the or keyword.
4. Negation: defines a conjunctive pattern that explicitly defines conditions not to be met. We form negations by defining the pattern of interest inside a not {}; block.

To better illustrate the possibilities, we will now look at an example of an expressive pattern.

The pattern above describes pairs of instances of person who are married, went to the same school and are employed by the same organisation. The pattern additionally specifies the employer to be either Pharos or Cybersafe, and the school to not be named HCC. Additionally the pattern asks to fetch the full-name of each of the people in the pair.

The pattern is a conjunction of four different pattern types:

• Conjunction 1 specifies the variables for people, school and organisation, specifies their types and asks for full-names of people.
• Disjunction specifies that the companies of interest are either Pharos or Cybersafe.
• Negation specifies that we are not interested in the people who attended the school named HCC.
• Conjunction 2 defines the pattern requiring the people to be in a marriage relationship, attend the same school via the studentship relationship, and work at the same organisation via the employment relationship.