CakePHP - 使用JOIN进行多表联合查询

在 SELECT, UPDATE 和 DELETE 语句中可以使用 JOIN 来进行多表联合查询，JOIN按照功能分为以下三类：

• INNER JOIN (内连接或等值连接)：获取两个表中字段匹配关系的记录
• LEFT JOIN (左连接)：获取左表所有记录，即使右表没有对应匹配的记录
• RIGHT JOIN (右连接)：与 LEFT JOIN 相反，用于获取右表所有记录，即使左表没有对应匹配的记录

例如有以下三张数据表，它们之间没有关联关系，但都有city_code 字段：

• 城市基础表 cities
• 城市信息表 city_infos
• 城市经济指标表 city_indicators

现需要从三张数据表中联合查询，原始SQL语句如下：

SELECT a.city_name, b.city_population, c.city_gdp
FROM cities a
INNER JOIN city_infos b
ON (
	a.city_code = b.city_code
	AND b.modified_year = '2020'
)
INNER JOIN city_indicators c
ON (
	a.city_code = c.city_code
	AND c.modified_year = '2020'
)
ORDER BY a.city_code ASC

在CakePHP的Controller类中实现以上联合查询，方式如下：

$this->Cities->find()
					->select([
						'Cities.city_name', 
						'CityInfos.city_population',
						'CityIndicators.city_gdp'
					])
					->innerJoin(
						[
							'CityInfos' => 'city_infos' //设置表的别名
						],
						[
							'Cities.city_code = CityInfos.city_code',
							'CityInfos.modified_year' => '2020'
						]
					)
					->innerJoin(
						[
							'CityIndicators' => 'city_indicators' //设置表的别名
						],
						[
							'Cities.city_code = CityIndicators.city_code',
							'CityIndicators.modified_year' => '2020'
						]
					)
					->order([
						'Cities.city_code' => 'ASC'
					])
					->map(function($row) { //构建数据格式
						return [
							'city_name' => $row->city_name,
							'city_population' => $row->CityInfos['city_population'],
							'city_gdp' => $row->CityIndicators['city_gdp']
						];
					})
					->toArray();

或者把所有要联合的表写在一起：

$this->Cities->find()
					->select([
						'Cities.city_name', 
						'CityInfos.city_population',
						'CityIndicators.city_gdp'
					])
					->join([
						'CityInfos' => [ //设置表的别名
							'table' => 'city_infos',
							'type' => 'INNER',
							'conditions' => [
								'CityInfos.city_code = Cities.city_code',
								'CityInfos.modified_year' => '2020'
							]
						],
						'CityIndicators' => [ //设置表的别名
							'table' => 'city_indicators',
							'type' => 'INNER',
							'conditions' => [
								'CityIndicators.city_code = Cities.city_code',
								'CityIndicators.modified_year' => '2020'
							]
						]
					])
					->order([
						'Cities.city_code' => 'ASC'
					])
					->map(function($row) { //构建数据格式
						return [
							'city_name' => $row->city_name,
							'city_population' => $row->CityInfos['city_population'],
							'city_gdp' => $row->CityIndicators['city_gdp']
						];
					})
					->toArray();