Translate the following question to SQL: What are the codes corresponding to document types for which there are less than 3 documents?

Schema: CREATE TABLE Ref_Document_Types (
    Document_Type_Code CHAR(15) NOT NULL  PRIMARY KEY,
    Document_Type_Name VARCHAR(255) NOT NULL,
    Document_Type_Description VARCHAR(255) NOT NULL,
    UNIQUE (Document_Type_Code)
);

CREATE TABLE Ref_Budget_Codes (
    Budget_Type_Code CHAR(15) NOT NULL  PRIMARY KEY,
    Budget_Type_Description VARCHAR(255) NOT NULL,
    UNIQUE (Budget_Type_Code)
);

CREATE TABLE Projects (
    Project_ID INTEGER NOT NULL  PRIMARY KEY,
    Project_Details VARCHAR(255)
);

CREATE TABLE Documents (
    Document_ID INTEGER NOT NULL  PRIMARY KEY,
    Document_Type_Code CHAR(15) NOT NULL,
    Project_ID INTEGER NOT NULL,
    Document_Date DATETIME,
    Document_Name VARCHAR(255),
    Document_Description VARCHAR(255),
    Other_Details VARCHAR(255),
    FOREIGN KEY (Project_ID) REFERENCES Projects(Project_ID),
    FOREIGN KEY (Document_Type_Code) REFERENCES Ref_Document_Types(Document_Type_Code)
);

CREATE TABLE Statements (
    Statement_ID INTEGER NOT NULL  PRIMARY KEY,
    Statement_Details VARCHAR(255),
    FOREIGN KEY (Statement_ID) REFERENCES Documents(Document_ID)
);

CREATE TABLE Documents_with_Expenses (
    Document_ID INTEGER NOT NULL  PRIMARY KEY,
    Budget_Type_Code CHAR(15) NOT NULL,
    Document_Details VARCHAR(255),
    FOREIGN KEY (Document_ID) REFERENCES Documents(Document_ID),
    FOREIGN KEY (Budget_Type_Code) REFERENCES Ref_Budget_Codes(Budget_Type_Code)
);

CREATE TABLE Accounts (
    Account_ID INTEGER NOT NULL  PRIMARY KEY,
    Statement_ID INTEGER NOT NULL,
    Account_Details VARCHAR(255),
    FOREIGN KEY (Statement_ID) REFERENCES Statements(Statement_ID)
);
user
SELECT document_type_code FROM Documents GROUP BY document_type_code HAVING count(*)  <  3
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