SaveAsTable
//Api的解释
Saves the content of the `DataFrame` as the specified table.
*
* In the case the table already exists, behavior of this function depends on the
* save mode, specified by the `mode` function (default to throwing an exception).
* When `mode` is `Overwrite`, the schema of the `DataFrame` does not need to be
* the same as that of the existing table.
*
* When `mode` is `Append`, if there is an existing table, we will use the format and options of
* the existing table. The column order in the schema of the `DataFrame` doesnt need to be same
* as that of the existing table. Unlike `insertInto`, `saveAsTable` will use the column names to
* find the correct column positions.
意思就是说,当hive中已经存在目标表，无论SaveMode是append还是overwrite，不需要schema一样,只要列名存在就行,会根据列名进行匹配覆盖数据
举个例子：
-----当hive中列名是i,j时
scala> Seq((1, 2)).toDF("i", "j").write.mode("overwrite").saveAsTable("t1")
* scala> Seq((3, 4)).toDF("j", "i").write.mode("append").saveAsTable("t1")
* scala> sql("select * from t1").show
* +---+---+
* | i| j|
* +---+---+
* | 1| 2|
* | 4| 3|
* +---+---+
InsertInto
//Api解释
Inserts the content of the `DataFrame` to the specified table. It requires that
* the schema of the `DataFrame` is the same as the schema of the table.
*
* @note Unlike `saveAsTable`, `insertInto` ignores the column names and just uses position-based
* resolution. For example:
意思就是说,当hive中存在目标表时,无论SaveMode是append还是overwrite，需要当前DF的schema与目标表的schema必须一致
举个例子：
-----当hive中列名是i,j时，schema为（int,int）
* {
{
{
* scala> Seq((1, 2)).toDF("i", "j").write.mode("overwrite").saveAsTable("t1")
* scala> Seq((3, 4)).toDF("j", "i").write.insertInto("t1")
* scala> Seq((5, 6)).toDF("a", "b").write.insertInto("t1")
* scala> sql("select * from t1").show
* +---+---+
* | i| j|
* +---+---+
* | 5| 6|
* | 3| 4|
* | 1| 2|
* +---+---+
* }}}
