Slick 编程(6): 查询(一)

本篇介绍Slick的基本查询，比如选择，插入，更新，删除记录等。
排序和过滤
Slick提供了多种方法可以用来排序和过滤，比如：

val q = Album.filter(_.albumid === 101)
 
//select `AlbumId`, `Title`, `ArtistId` 
//from `Album` where `AlbumId` = 101
 
 
val q = Album.drop(10).take(5)
//select .`AlbumId` as `AlbumId`, .`Title` as `Title`,
// .`ArtistId` as `ArtistId` from `Album`  limit 10,5
 
 
val q = Album.sortBy(_.title.desc)
//select `AlbumId`, `Title`, `ArtistId` 
//from `Album` order by `Title` desc

Join和Zipping
Join指多表查询，可以有两种不同的方法来实现多表查询，一种是通过明确调用支持多表连接的方法(比如innerJoin方法)返回一个多元组，另外一种为隐含连接(implicit join)，它不直接使用这些连接方法（比如LeftJoin方法）。
一个隐含的cross-Join 为Query的flatMap操作（在for表达式中使用多个生成式），例如：

val q = for{a <- Album
			b <- Artist
		} yield( a.title, b.name)
 
//select x2.`Title`, x3.`Name` from `Album` x2, `Artist` x3

如果添加一个条件过滤表达式，它就变成隐含的inner join，例如：

val q = for{a <- Album
			b <- Artist
		    if a.artistid === b.artistid
		} yield( a.title, b.name)
 
//select x2.`Title`, x3.`Name` from `Album` x2, `Artist` x3 
//where x2.`ArtistId` = x3.`ArtistId`
 

明确的多表连接则使用innerJoin , leftJoin ,rightJoin,outerJoin 方法，例如：

val explicitCrossJoin = = for {
			 (a,b) <- Album innerJoin Artist  
			 } yield( a.title, b.name)
 
 
//select x2.x3, x4.x5 from (select x6.`Title` as x3 from `Album` x6) 
//x2 inner join (select x7.`Name` as x5 from `Artist` x7) x4 on 1=1
 
 
 val explicitInnerJoin  = for {
		 (a,b) <- Album innerJoin Artist on (_.artistid === _.artistid)
		 } yield( a.title, b.name)
//select x2.x3, x4.x5 from (select x6.`Title` as x3, x6.`ArtistId` as x7 from `Album` x6) x2 
//inner join (select x8.`ArtistId` as x9, x8.`Name` as x5 from `Artist` x8) x4 on x2.x7 = x4.x9
 
 
val explicitLeftOuterJoin   = for {
		 (a,b) <- Album leftJoin Artist on (_.artistid === _.artistid)
		 } yield( a.title, b.name.?)
//select x2.x3, x4.x5 from (select x6.`Title` as x3, x6.`ArtistId` as x7 from `Album` x6) x2 
//left outer join (select x8.`ArtistId` as x9, x8.`Name` as x5 from `Artist` x8) x4 on x2.x7 = x4.x9
 
 
val explicitRightOuterJoin   = for {
		 (a,b) <- Album rightJoin Artist on (_.artistid === _.artistid)
		 } yield( a.title.?, b.name)
//select x2.x3, x4.x5 from (select x6.`Title` as x3, x6.`ArtistId` as x7 from `Album` x6) x2 
//right outer join (select x8.`ArtistId` as x9, x8.`Name` as x5 from `Artist` x8) x4 on x2.x7 = x4.x9
 

注意leftJoin 和 rightJoin中的 b.name.?和 a.title.? 的?.?? 这是因为外部查询时会产生额外的NULL值，你必须保证返回Option类型的值。
除了通常的InnerJoin ，LeftJoin,RightJoin之外，Scala还提供了Zip 方法，它的语法类似于Scala的集合类型，比如：

val zipJoinQuery  = for {
	   (a,b) <- Album zip Artist
	 } yield( a.title.?, b.name)
 

此外，还有一个zipWithIndex，可以把一个表的行和一个从0开始的整数序列Zip操作，相当于给行添加序号，比如

val zipWithIndexJoin  = for {
	   (a,idx) <- Album.zipWithIndex 
	 } yield( a.title, idx)