scala - Count rows with Slick 1.0.0

Question

I'm trying to create a query with Slick 1.0.0 that returns a row count equivalent to the following SQL statement:

SELECT COUNT(*) FROM table;

What I have so far is:

val query = for {
  row <- Table
} yield row
println(query.length)

This prints scala.slick.ast.FunctionSymbol$$anon$1@6860991f. Also, query.length appears to be of type scala.slick.lifted.Column. I cannot find a way to execute the query. All examples that I can find in the documentation and anywhere else do not operate on Column or are for ScalaQuery and do not work anymore.

What can I do to execute this?

Answer 1

虽然我无法检查生成的 sql，但您可以通过删除 .list 来获得更短的源代码：

Query(MyTable.length).first

Answer 2

这些中的任何一个都可以解决问题：

Query(MyTable).list.length

或者

(for{mt <- MyTable} yield mt).list.length

或者

(for{mt <- MyTable} yield mt.count).first

更新：

打印 H2 数据库日志显示了最后一个查询，这看起来是最佳的：

 03:31:26.560 [main] DEBUG h2database - jdbc[2]
 /**/PreparedStatement prep10 = conn1.prepareStatement("select select count(1) from \"MYTABLE\" s5", 1003, 1007);

Answer 3

利用：

val query = for(row <- Table) yield row 
println(Query(query.count).first)

count相当于“SELECT COUNT(*) FROM Table” 。为了获得第一行也是唯一的行，您必须使用它first来获取计数。