列映射

视图对象的字段不仅用于映射不同表的列，还用于表示聚合表达式和分组子句。

聚合列映射

聚合函数的应用是聚合查询的核心功能。所有通过聚合列查询的数据都需由视图类中定义的字段保存，因此我们需要将字段名映射为聚合列，使用前缀映射是一种很好的选择。

前缀	聚合函数名	字段名	聚合列表达式
sum	sum	sumScore	sum(score) AS sumScore
max	max	maxScore	max(score) AS maxScore
min	min	minScore	min(score) AS minScore
avg	avg	avgScore	avg(score) AS avgScore
first	first	firstScore	first(score) AS firstScore
last	last	lastScore	last(score) AS lastScore
stdDevPop	stddev_pop	stdDevPopScore	stddev_pop(score) AS stdDevPopScore
stdDevSamp	stddev_samp	stdDevSampScore	stddev_samp(score) AS stdDevSampScore
stdDev	stddev	stdDev	stddev(score) AS stdDev
addToSet	addToSet	addToSetScore	addToSet(score) AS addToSetScore
push	push	pushScore	push(score) AS pushScore
count	count	countScore	count(score) AS countScore
count	count	count	count(*) AS count

示例 如果要计算名为 score 的列的平均值，使用 avg 函数，定义字段名为 avgScore，遵循聚合前缀+列名的命名规则，最终映射为 avg(score) AS avgScore。

聚合表达式映射

对于无法直接用字段名表示的聚合表达式，可以通过注解指定具体的表达式，将表达式映射为列名，字段名作为标签。

示例 定义字段时添加表达式注解：

@Column(name = "sum(l_extendedprice*(1-l_discount))")
private BigDecimal sum_disc_price;

该字段映射为：

sum(l_extendedprice*(1-l_discount)) AS sum_disc_price

完整示例 请参见表格：

LN	对象代码	SQL 语句
1	@View(CustomerEntity.class)	SELECT
2	@View(OrdersEntity.class)
3	@View(LineitemEntity.class)
4	public class ShippingPriorityView {
5	@GroupBy
6	private String l_orderkey;	l_orderkey,
7	@Column(name = "SUM(l_extendedprice * (1 - l_discount))")	SUM(l_extendedprice * (1 - l_discount)) AS revenue,
8	private Double revenue;
9	@GroupBy
10	private Date o_orderdate;	o_orderdate,
11	@GroupBy
12	private String o_shippriority;	o_shippriority
13	}	FROM customer, orders, lineitem
	// @View(CustomerEntity.class)	WHERE o_custkey = c_custkey
	// @View(OrdersEntity.class)	AND l_orderkey = o_orderkey
	// @View(LineitemEntity.class)
1	public class ShippingPriorityQuery extends PageQuery {
2	private String c_mktsegment;	AND c_mktsegment = ?
3	private Date o_orderdateLt;	AND o_orderdate < ?
4	private Date l_shipdateGt;	AND l_shipdate > ?
5	}	GROUP BY l_orderkey, o_orderdate, o_shippriority
	// PageQuery.sort = "revenue,DESC;o_orderdate"	ORDER BY revenue DESC, o_orderdate