Implementing LIKE Operator

[syedsafi30]

Overview
The backend changes required to implement LIKE Operator, similar to SQL (As explained in issue #310) in JAVA are included in this PR.

Changes Included

1. Added Conditional Statements for executing the Pattern matching conditions.
2. Added Switch-Case Statements which perform different functionalities of LIKE Operator.

[nilshansen94]

Hi @syedsafi30. If I understand correctly, your implementation of the LIKE operator returns always true. The implementation did not solve the problems described in the issue. The first goal is to create a regular expression where all necessary characters are escaped (for example a dot should be escaped). The second goal is to provide support for the % and _ wildcards, as described in the link in the issue.
Does your implementation of the FIRST/LAST/CONTAINS operators work? If yes, could you provide an example (SQL code) on how to use these operators?
To make an example, the LIKE 'a..bc%d_ef' should result in the regex a\\.\\.bc.*d.ef or even better a\Q..\Ebc.*d.ef.

[syedsafi30]

Hi @nilshansen94 , thanks for your response.
You are absolutely right, the LIKE operator which is implemented in the code returns true only if the pattern matches with the other (if the two strings are alike). The other operations of like Operator are performed by CONTAINS operator. It checks if a particular string contains a certain pattern(as we check for a pattern in SQL by entering %abe.whe% to check in a string. The CONTAINS operator does this task and returns true/false. The rest of the operations to check if a regex starts with or ends with a certain pattern are performed by FIRST & LAST operator( i suppose this can be done by using RIGHT & LEFT operator too).

I’ll be happy to take suggestions and improve the code further.

[nilshansen94]

Hi @syedsafi30, thank you for your work. Your code does not work unfortunately. As mentioned before, the code in the case LIKE: ... always returns true;
To test your implementation, please make sure to work with a table that is placed on a file store only. Here is an example SQL code on how to set up such a table. There is also an example of a LIKE query and the expected result. The implementation should work in the same way as it does with a hsqldb table.

ALTER ADAPTERS ADD "file" USING 'org.polypheny.db.adapter.file.FileStore' WITH '{}';
CREATE TABLE "public"."user"("id" BIGINT NOT NULL,"name" VARCHAR(20),"comment" VARCHAR(174),PRIMARY KEY ("id")) ON STORE "file";
INSERT INTO public."user" (id, name, comment) values (1, 'anna', 'Hello world, how are you'), (2, 'bob', 'Hello World, how ore ya'), (3, 'john', 'hi');
select * from "user" where comment like 'Hell_ wo%'; --matches only the row with anna, because we're matching case-sensitive

The idea is that the query string in the parameterValue should be escaped and that the % should be replaced with a .* in the regular expression and the _ should be replaced with .. In the example, the parameterValue Hell_ wo% should be replaced with the regular expression Hell. wo.*. In my previous comment I made an example on how to escape special characters.

[syedsafi30]

Hi @nilshansen94 Thanks for your briefed response, I have started writing an algorithm towards resolving the issue to implement the LIKE operator and the wildcards as you have described above.

[syedsafi30]

Hi @nilshansen94, I have made the necessary changes to the file, please review them.