[BE/#17] Feat : LabelDao 쿼리 설계

- 모든 레이블 목록을 반환하는 쿼리 추가
- 해당 이슈에 할당된 레이블을 반환하는 쿼리 추가
- SQL을 최대한 작은 단위로 활용

BE/src/main/java/com/codesquad/issuetracker/ragdoll/dao/LabelDao.java

@@ -0,0 +1,34 @@
+package com.codesquad.issuetracker.ragdoll.dao;
+
+import com.codesquad.issuetracker.ragdoll.domain.Label;
+import com.codesquad.issuetracker.ragdoll.dto.labelVO.LabelSummary;
+import org.springframework.jdbc.core.JdbcTemplate;
+import org.springframework.stereotype.Repository;
+
+import javax.sql.DataSource;
+import java.util.List;
+
+@Repository
+public class LabelDao {
+
+    private JdbcTemplate jdbcTemplate;
+
+    public LabelDao(DataSource dataSource) {
+        this.jdbcTemplate = new JdbcTemplate(dataSource);
+    }
+
+    public List<LabelSummary> findAttachedLabelsByIssueId(Long issueId) {
+        String sql = "SELECT l.id, l.name, l.hex_code " +
+                     "FROM label l JOIN issue_has_label il ON l.id = il.label_id " +
+                     "WHERE il.issue_id = ?";
+        return jdbcTemplate.query(sql, new Object[]{issueId},
+                (rs, rowNum) -> LabelSummary.create(rs.getInt("l.id"), rs.getString("l.name"),
+                                                    rs.getString("l.hex_code")));
+    }
+
+    public List<Label> findAllLabels() {
+        String sql = "SELECT id, name, description, hex_code FROM label";
+        return jdbcTemplate.query(sql, (rs, rowNum) -> Label.create(rs.getInt("id"), rs.getString("name"),
+                                                                    rs.getString("description"), rs.getString("hex_code")));
+    }
+}