Scoop是用来实现HDFS文件系统和关系型数据库如MySQL之间数据传输和转换的工具。

从MySQL导出到HDFS可以通过`--table`, `--columns` and `--where`等设置数据抽出的条件。但是同时也只是自由sql语句（Free-form Query ）的方式抽出数据。此时我们用--query加sql语句方式自由抽取数据。

1，必须制定目标文件的位置--target-dir

2，必须使用$CONDITIONS关键字，

3，你也可以选择使用--split-by分片(分区，结果分成多个小文件，请参考mapreduce分区)

我们主要讨论`$CONDITIONS关键字的作用是什么。`

1如果直接输出，这里面是空的条件

2，我们在执行log中发现被替换成了1=0

sqoop import   --connect jdbc:mysql://server74:3306/Server74   --username root  --password 123456  --target-dir /sqoopout2  --m 1 --delete-target-dir 
 --query 'select id,name,deg from emp where id>1202 and $CONDITIONS'

[root@server72 sqoop]# sqoop import   --connect jdbc:mysql://server74:3306/Server74   --username root  --password 123456  --target-dir /sqoopout2  
--m 1 --delete-target-dir  --query 'select id,name,deg from emp where id>1202 and $CONDITIONS' 
Warning: /usr/local/sqoop/../hbase does not exist! HBase imports will fail.
Please set $HBASE_HOME to the root of your HBase installation.
Warning: /usr/local/sqoop/../hcatalog does not exist! HCatalog jobs will fail.
Please set $HCAT_HOME to the root of your HCatalog installation.
Warning: /usr/local/sqoop/../accumulo does not exist! Accumulo imports will fail.
Please set $ACCUMULO_HOME to the root of your Accumulo installation.
17/11/10 13:42:14 INFO sqoop.Sqoop: Running Sqoop version: 1.4.6
17/11/10 13:42:14 WARN tool.BaseSqoopTool: Setting your password on the command-line is insecure. Consider using -P instead.
17/11/10 13:42:16 INFO manager.MySQLManager: Preparing to use a MySQL streaming resultset.
17/11/10 13:42:16 INFO tool.CodeGenTool: Beginning code generation
17/11/10 13:42:18 INFO manager.SqlManager: Executing SQL statement: select id,name,deg from emp where id>1202 and  (1 = 0) 
17/11/10 13:42:18 INFO manager.SqlManager: Executing SQL statement: select id,name,deg from emp where id>1202 and  (1 = 0) 
17/11/10 13:42:18 INFO manager.SqlManager: Executing SQL statement: select id,name,deg from emp where id>1202 and  (1 = 0) 
17/11/10 13:42:18 INFO orm.CompilationManager: HADOOP_MAPRED_HOME is /usr/local/hadoop
Note: /tmp/sqoop-root/compile/ac7745794cf5f0bf5859e7e8369a8c5f/QueryResult.java uses or overrides a deprecated API.
Note: Recompile with -Xlint:deprecation for details.
17/11/10 13:42:31 INFO orm.CompilationManager: Writing jar file: /tmp/sqoop-root/compile/ac7745794cf5f0bf5859e7e8369a8c5f/QueryResult.jar
17/11/10 13:42:33 WARN util.NativeCodeLoader: Unable to load native-hadoop library for your platform... using builtin-java classes where applicable
17/11/10 13:42:41 INFO tool.ImportTool: Destination directory /sqoopout2 deleted.
17/11/10 13:42:41 INFO mapreduce.ImportJobBase: Beginning query import.
17/11/10 13:42:41 INFO Configuration.deprecation: mapred.jar is deprecated. Instead, use mapreduce.job.jar
17/11/10 13:42:41 INFO Configuration.deprecation: mapred.map.tasks is deprecated. Instead, use mapreduce.job.maps
17/11/10 13:42:43 INFO client.RMProxy: Connecting to ResourceManager at server71/192.168.32.71:8032
17/11/10 13:42:58 INFO db.DBInputFormat: Using read commited transaction isolation
17/11/10 13:42:58 INFO mapreduce.JobSubmitter: number of splits:1
17/11/10 13:43:00 INFO mapreduce.JobSubmitter: Submitting tokens for job: job_1510279795921_0011
17/11/10 13:43:03 INFO impl.YarnClientImpl: Submitted application application_1510279795921_0011
17/11/10 13:43:04 INFO mapreduce.Job: The url to track the job: http://server71:8088/proxy/application_1510279795921_0011/
17/11/10 13:43:04 INFO mapreduce.Job: Running job: job_1510279795921_0011
17/11/10 13:44:01 INFO mapreduce.Job: Job job_1510279795921_0011 running in uber mode : false
17/11/10 13:44:01 INFO mapreduce.Job:  map 0% reduce 0%
17/11/10 13:44:58 INFO mapreduce.Job:  map 100% reduce 0%
17/11/10 13:45:00 INFO mapreduce.Job: Job job_1510279795921_0011 completed successfully
17/11/10 13:45:01 INFO mapreduce.Job: Counters: 30
    File System Counters
        FILE: Number of bytes read=0
        FILE: Number of bytes written=124473
        FILE: Number of read operations=0
        FILE: Number of large read operations=0
        FILE: Number of write operations=0
        HDFS: Number of bytes read=87
        HDFS: Number of bytes written=61
        HDFS: Number of read operations=4
        HDFS: Number of large read operations=0
        HDFS: Number of write operations=2
    Job Counters 
        Launched map tasks=1
        Other local map tasks=1
        Total time spent by all maps in occupied slots (ms)=45099
        Total time spent by all reduces in occupied slots (ms)=0
        Total time spent by all map tasks (ms)=45099
        Total vcore-milliseconds taken by all map tasks=45099
        Total megabyte-milliseconds taken by all map tasks=46181376
    Map-Reduce Framework
        Map input records=3
        Map output records=3
        Input split bytes=87
        Spilled Records=0
        Failed Shuffles=0
        Merged Map outputs=0
        GC time elapsed (ms)=370
        CPU time spent (ms)=6380
        Physical memory (bytes) snapshot=106733568
        Virtual memory (bytes) snapshot=842854400
        Total committed heap usage (bytes)=16982016
    File Input Format Counters 
        Bytes Read=0
    File Output Format Counters 
        Bytes Written=61
17/11/10 13:45:01 INFO mapreduce.ImportJobBase: Transferred 61 bytes in 139.3429 seconds (0.4378 bytes/sec)
17/11/10 13:45:01 INFO mapreduce.ImportJobBase: Retrieved 3 records.

输出结果查看，发现1202以上的数据被正常抽出
[root@server72 sqoop]# hdfs dfs -cat /sqoopout2/part-m-00000
17/11/10 13:48:48 WARN util.NativeCodeLoader: Unable to load native-hadoop library for your platform... using builtin-java classes where applicable
1203,khalil,php dev
1204,prasanth,php dev
1205,kranthi,admin

通过以上过程，我们得知一点：$CONTITONS是linux系统的变量，在执行过程中被赋值为（1=0），虽然实际执行的这个sql很奇怪。

现在正式开始研究CONTITONS到底是什么，所以我们先查看官方文档。

If you want to import the results of a query in parallel, then each map task will need to execute a copy of the query, with results partitioned by bounding conditions inferred by Sqoop. Your query must include the token $CONDITIONS which each Sqoop process will replace with a unique condition expression. You must also select a splitting column with --split-by.

如果你想通过并行的方式导入结果，每个map task需要执行sql查询语句的副本，结果会根据sqoop推测的边界条件分区。query必须包含$CONDITIONS。这样每个scoop程序都会被替换为一个独立的条件。同时你必须指定--split-by.分区

For example:
$ sqoop import \

  --query 'SELECT a.*, b.* FROM a JOIN b on (a.id == b.id) WHERE $CONDITIONS' \

  --split-by a.id --target-dir /user/foo/joinresults

直接理解可能有点困难，我先修改一些条件，大家观察joblog的区别。

sqoop import --connect jdbc:mysql://server74:3306/Server74 --username root --password 123456 --target-dir /sqoopout2

　　　　　　--m 2 --delete-target-dir --query 'select id,name,deg from emp where id>1202 and $CONDITIONS'

　　　　　　--split-by id

我按照要求添加了--split-by id 分区，并设置map task数量为2

[root@server72 sqoop]# sqoop import   --connect jdbc:mysql://server74:3306/Server74   --username root 
 --password 123456  --target-dir /sqoopout2  --m 2 --delete-target-dir  --query 'select id,name,deg from emp where id>1202 and $CONDITIONS' --split-by id

Warning: /usr/local/sqoop/../hbase does not exist! HBase imports will fail.

Please set $HBASE_HOME to the root of your HBase installation.

Warning: /usr/local/sqoop/../hcatalog does not exist! HCatalog jobs will fail.

Please set $HCAT_HOME to the root of your HCatalog installation.

Warning: /usr/local/sqoop/../accumulo does not exist! Accumulo imports will fail.

Please set $ACCUMULO_HOME to the root of your Accumulo installation.

// :: INFO sqoop.Sqoop: Running Sqoop version: 1.4.

// :: WARN tool.BaseSqoopTool: Setting your password on the command-line is insecure. Consider using -P instead.

// :: INFO manager.MySQLManager: Preparing to use a MySQL streaming resultset.

// :: INFO tool.CodeGenTool: Beginning code generation

// :: INFO manager.SqlManager: Executing SQL statement: select id,name,deg from emp where id> and  ( = )

// :: INFO manager.SqlManager: Executing SQL statement: select id,name,deg from emp where id> and  ( = )

// :: INFO manager.SqlManager: Executing SQL statement: select id,name,deg from emp where id> and  ( = )

// :: INFO orm.CompilationManager: HADOOP_MAPRED_HOME is /usr/local/hadoop

Note: /tmp/sqoop-root/compile/1024341fa58082466565e5bd648cb10e/QueryResult.java uses or overrides a deprecated API.

Note: Recompile with -Xlint:deprecation for details.

// :: INFO orm.CompilationManager: Writing jar file: /tmp/sqoop-root/compile/1024341fa58082466565e5bd648cb10e/QueryResult.jar

// :: WARN util.NativeCodeLoader: Unable to load native-hadoop library for your platform... using builtin-java classes where applicable

// :: INFO tool.ImportTool: Destination directory /sqoopout2 deleted.

// :: INFO mapreduce.ImportJobBase: Beginning query import.

// :: INFO Configuration.deprecation: mapred.jar is deprecated. Instead, use mapreduce.job.jar

// :: INFO Configuration.deprecation: mapred.map.tasks is deprecated. Instead, use mapreduce.job.maps

// :: INFO client.RMProxy: Connecting to ResourceManager at server71/192.168.32.71:

// :: INFO db.DBInputFormat: Using read commited transaction isolation

// :: INFO db.DataDrivenDBInputFormat: BoundingValsQuery: SELECT MIN(id), MAX(id) FROM (select id,name,deg from emp where id>1202 and  (1 = 1) ) AS t1

// :: INFO mapreduce.JobSubmitter: number of splits:

// :: INFO mapreduce.JobSubmitter: Submitting tokens for job: job_1510279795921_0012

// :: INFO impl.YarnClientImpl: Submitted application application_1510279795921_0012

// :: INFO mapreduce.Job: The url to track the job: http://server71:8088/proxy/application_1510279795921_0012/

// :: INFO mapreduce.Job: Running job: job_1510279795921_0012

// :: INFO mapreduce.Job: Job job_1510279795921_0012 running in uber mode : false

// :: INFO mapreduce.Job:  map % reduce %

// :: INFO mapreduce.Job:  map % reduce %

// :: INFO mapreduce.Job:  map % reduce %

// :: INFO mapreduce.Job:  map % reduce %

// :: INFO mapreduce.Job: Job job_1510279795921_0012 completed successfully

// :: INFO mapreduce.Job: Counters:

    File System Counters

        FILE: Number of bytes read=

        FILE: Number of bytes written=

        FILE: Number of read operations=

        FILE: Number of large read operations=

        FILE: Number of write operations=

        HDFS: Number of bytes read=

        HDFS: Number of bytes written=

        HDFS: Number of read operations=

        HDFS: Number of large read operations=

        HDFS: Number of write operations=

    Job Counters

        Killed map tasks=

        Launched map tasks=

        Other local map tasks=

        Total time spent by all maps in occupied slots (ms)=

        Total time spent by all reduces in occupied slots (ms)=

        Total time spent by all map tasks (ms)=

        Total vcore-milliseconds taken by all map tasks=

        Total megabyte-milliseconds taken by all map tasks=

    Map-Reduce Framework

        Map input records=

        Map output records=

        Input split bytes=

        Spilled Records=

        Failed Shuffles=

        Merged Map outputs=

        GC time elapsed (ms)=

        CPU time spent (ms)=

        Physical memory (bytes) snapshot=

        Virtual memory (bytes) snapshot=

        Total committed heap usage (bytes)=

    File Input Format Counters

        Bytes Read=

    File Output Format Counters

        Bytes Written=

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Sqoop--Free-form Query Imports 自由查询模式下$CONDITIONS关键字的作用