mysql 错误：java.sql.SQLException: Unknown system variable 'language'

报错问题如下：
java.sql.SQLException: Unknown system variable 'language'
	at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:965) ~[mysql-connector-java-5.1.46.jar:5.1.46]
	at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3976) ~[mysql-connector-java-5.1.46.jar:5.1.46]
	at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3912) ~[mysql-connector-java-5.1.46.jar:5.1.46]
	at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:2530) ~[mysql-connector-java-5.1.46.jar:5.1.46]
	at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:2683) ~[mysql-connector-java-5.1.46.jar:5.1.46]
	at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2482) ~[mysql-connector-java-5.1.46.jar:5.1.46]
	at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2440) ~[mysql-connector-java-5.1.46.jar:5.1.46]
	at com.mysql.jdbc.StatementImpl.executeQuery(StatementImpl.java:1381) ~[mysql-connector-java-5.1.46.jar:5.1.46]
	at com.mysql.jdbc.ConnectionImpl.loadServerVariables(ConnectionImpl.java:3786) ~[mysql-connector-java-5.1.46.jar:5.1.46]
	at com.mysql.jdbc.ConnectionImpl.initializePropsFromServer(ConnectionImpl.java:3221) ~[mysql-connector-java-5.1.46.jar:5.1.46]
	at com.mysql.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:2234) ~[mysql-connector-java-5.1.46.jar:5.1.46]
	at com.mysql.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:2016) ~[mysql-connector-java-5.1.46.jar:5.1.46]
	at com.mysql.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:776) ~[mysql-connector-java-5.1.46.jar:5.1.46]
	at com.mysql.jdbc.JDBC4Connection.<init>(JDBC4Connection.java:47) ~[mysql-connector-java-5.1.46.jar:5.1.46]
	at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[na:1.8.0_111]
	at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62) ~[na:1.8.0_111]
	at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) ~[na:1.8.0_111]
	at java.lang.reflect.Constructor.newInstance(Constructor.java:423) ~[na:1.8.0_111]
	at com.mysql.jdbc.Util.handleNewInstance(Util.java:425) ~[mysql-connector-java-5.1.46.jar:5.1.46]
	at com.mysql.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:386) ~[mysql-connector-java-5.1.46.jar:5.1.46]
	at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:330) ~[mysql-connector-java-5.1.46.jar:5.1.46]
	at com.zaxxer.hikari.util.DriverDataSource.getConnection(DriverDataSource.java:117) ~[HikariCP-2.7.9.jar:na]
	at com.zaxxer.hikari.util.DriverDataSource.getConnection(DriverDataSource.java:123) ~[HikariCP-2.7.9.jar:na]
	at com.zaxxer.hikari.pool.PoolBase.newConnection(PoolBase.java:365) ~[HikariCP-2.7.9.jar:na]
	at com.zaxxer.hikari.pool.PoolBase.newPoolEntry(PoolBase.java:194) ~[HikariCP-2.7.9.jar:na]
	at com.zaxxer.hikari.pool.HikariPool.createPoolEntry(HikariPool.java:460) [HikariCP-2.7.9.jar:na]
	at com.zaxxer.hikari.pool.HikariPool.checkFailFast(HikariPool.java:534) [HikariCP-2.7.9.jar:na]
	at com.zaxxer.hikari.pool.HikariPool.<init>(HikariPool.java:115) [HikariCP-2.7.9.jar:na]
	at com.zaxxer.hikari.HikariDataSource.getConnection(HikariDataSource.java:112) [HikariCP-2.7.9.jar:na]
	at org.springframework.jdbc.datasource.DataSourceUtils.fetchConnection(DataSourceUtils.java:151) [spring-jdbc-5.0.6.RELEASE.jar:5.0.6.RELEASE]
	at org.springframework.jdbc.datasource.DataSourceUtils.doGetConnection(DataSourceUtils.java:115) [spring-jdbc-5.0.6.RELEASE.jar:5.0.6.RELEASE]
	at org.springframework.jdbc.datasource.DataSourceUtils.getConnection(DataSourceUtils.java:78) [spring-jdbc-5.0.6.RELEASE.jar:5.0.6.RELEASE]
	at org.springframework.jdbc.core.JdbcTemplate.execute(JdbcTemplate.java:371) [spring-jdbc-5.0.6.RELEASE.jar:5.0.6.RELEASE]
	at org.springframework.jdbc.core.JdbcTemplate.update(JdbcTemplate.java:514) [spring-jdbc-5.0.6.RELEASE.jar:5.0.6.RELEASE]
	at com.example.demo.service.support.UserServiceImpl.deleteAllUsers(UserServiceImpl.java:28) [classes/:na]
	at com.example.demo.ApplicationTests.setUp(ApplicationTests.java:23) [test-classes/:na]
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_111]
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_111]
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_111]
	at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_111]
	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50) [junit-4.12.jar:4.12]
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12) [junit-4.12.jar:4.12]
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47) [junit-4.12.jar:4.12]
	at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:24) [junit-4.12.jar:4.12]
	at org.springframework.test.context.junit4.statements.RunBeforeTestMethodCallbacks.evaluate(RunBeforeTestMethodCallbacks.java:75) [spring-test-5.0.6.RELEASE.jar:5.0.6.RELEASE]
	at org.springframework.test.context.junit4.statements.RunAfterTestMethodCallbacks.evaluate(RunAfterTestMethodCallbacks.java:86) [spring-test-5.0.6.RELEASE.jar:5.0.6.RELEASE]
	at org.springframework.test.context.junit4.statements.SpringRepeat.evaluate(SpringRepeat.java:84) [spring-test-5.0.6.RELEASE.jar:5.0.6.RELEASE]
	at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325) [junit-4.12.jar:4.12]
	at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:251) [spring-test-5.0.6.RELEASE.jar:5.0.6.RELEASE]
	at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:97) [spring-test-5.0.6.RELEASE.jar:5.0.6.RELEASE]
	at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290) [junit-4.12.jar:4.12]
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71) [junit-4.12.jar:4.12]
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288) [junit-4.12.jar:4.12]
	at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58) [junit-4.12.jar:4.12]
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268) [junit-4.12.jar:4.12]
	at org.springframework.test.context.junit4.statements.RunBeforeTestClassCallbacks.evaluate(RunBeforeTestClassCallbacks.java:61) [spring-test-5.0.6.RELEASE.jar:5.0.6.RELEASE]
	at org.springframework.test.context.junit4.statements.RunAfterTestClassCallbacks.evaluate(RunAfterTestClassCallbacks.java:70) [spring-test-5.0.6.RELEASE.jar:5.0.6.RELEASE]
	at org.junit.runners.ParentRunner.run(ParentRunner.java:363) [junit-4.12.jar:4.12]
	at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.run(SpringJUnit4ClassRunner.java:190) [spring-test-5.0.6.RELEASE.jar:5.0.6.RELEASE]
	at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:86) [.cp/:na]
	at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38) [.cp/:na]
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:538) [.cp/:na]
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:760) [.cp/:na]
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:460) [.cp/:na]
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:206) [.cp/:na]
截图如下：
究其原因为数据库的pom配置或者引用的驱动jar包版本过高，而导致无法发现系统的变量。
解决方案如下：
首先，查看mysql版本：select version();
将查询到的版本与当前引用版本对比。看是否版本一致。如果不一致，调整为一致的版本即可。
mysql 错误：java.sql.SQLException: Unknown system variable 'language'

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