{"id":151,"date":"2019-03-19T08:30:13","date_gmt":"2019-03-19T08:30:13","guid":{"rendered":"https:\/\/www.kindsonthegenius.com\/spring-boot\/?p=151"},"modified":"2019-03-19T08:30:13","modified_gmt":"2019-03-19T08:30:13","slug":"spring-boot-query-methods","status":"publish","type":"post","link":"https:\/\/www.kindsonthegenius.com\/spring-boot\/spring-boot-query-methods\/","title":{"rendered":"Spring Boot – Query Methods"},"content":{"rendered":"

So now, we we are going to look at Query Methods. You use query methods to retrieve data from the repository. Query Methods are much easier to follow than SQL Queries.<\/p>\n

 <\/p>\n

We’ll cover the following:<\/p>\n

    \n
  1. Introduction to Query Methods<\/a><\/li>\n
  2. Values Returned from Query Methods<\/a><\/li>\n
  3. Passing Parameters to Query Methods<\/a><\/li>\n
  4. The Like Clause in Query Methods<\/a><\/li>\n
  5. Summary<\/a><\/li>\n<\/ol>\n

     <\/p>\n

     <\/p>\n

    1. Introduction to Query Methods<\/strong><\/h4>\n

    We use Query Methods to find information from the database. So you don’t need to learn SQL queries. Query methods are declared in the repository interface. For example, we have our Student class. It have member variables of id, name and department.\u00a0 Recall that we can use the method findById() of the repository interface to get student by id.<\/p>\n

    Similarly if we want to get student by name we can define findByName() method in the repository. JPA provides the implementation. Now, these are query methods that have been written for us.<\/p>\n

     <\/p>\n

    public<\/span> interface<\/span> StudentRepository<\/span> extends<\/span> CrudRepository<<\/span>Student,<\/span> String><\/span> {<\/span>\r\n\t\r\n    \/\/this is a query method already implemented by JPA<\/span>\r\n   public<\/span> Optional<<\/span>Student><\/span> findById<\/span>(<\/span>String id);<\/span>\r\n\t\r\n}<\/span>\r\n<\/pre>\n
     <\/p>\n
    I would recommend you try to change up things a bit and see what results you get.<\/p>\n
     <\/p>\n
     <\/p>\n
    2. Values Returned From Query Methods<\/strong><\/h4>\n
    Also recall that in these method return either a List<Object> or a single object. Generally, a Query method can return either one or more results.<\/p>\n
    Let’s now write a query method to select only the name. The code below shows what this method would be using query methods.<\/p>\n
     <\/p>\n
    @Query<\/span>(<\/span>\"SELECT t.name FROM Student t where t.id = :id\"<\/span>)<\/span>\r\npublic<\/span> Optional<<\/span>String><\/span> findNameById<\/span>(<\/span>@Param<\/span>(<\/span>\"id\"<\/span>)<\/span>String id);<\/span>\r\n<\/pre>\n
     <\/p>\n
    In this case, we are returning only the name of the Student where the id is given. This is very much similar to SQL Query.<\/p>\n
    Again, if we want to return the Student object, we simply replace the String return type with Student. We also replace t.name<\/em> with *. So we have the code below would return the Student with the given id.<\/p>\n
     <\/p>\n
    @Query<\/span>(<\/span>\"SELECT * FROM Student t where t.id = :id\"<\/span>)<\/span>\r\npublic<\/span> Optional<<\/span>Student><\/span> findById<\/span>(<\/span>@Param<\/span>(<\/span>\"id\"<\/span>)<\/span>String id);<\/span>\r\n<\/pre>\n
     <\/p>\n
    Now you can modify this queries to get any result you want. For example, you can do findByName() and so on. Let’s now talk about parameters.<\/p>\n
     <\/p>\n
     <\/p>\n
    3. Passing Parameters to Query Methods<\/strong><\/p>\n
    Now we would see how we can query with more than on criteria. For example, where name =”somename” and department=”somedepartment”. We can do this in two ways:<\/p>\n