1、Oracle版本：
1.1、执行语句：SELECT 'k' as a1,'a' as a2,'y' as a3FROM sr_main
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1 k a y
2 k a y
3 k a y
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1.2执行语句：SELECT concat( 'k','a') as pjFROM sr_main
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1 ka
2 ka
3 ka
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1.3执行语句：SELECT concat( 'k','a','y') as pjFROM sr_main
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Error：invalid number of arguments
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Reason：Concat只能连接两个字符串
1.4执行语句：SELECT 'k'||'a'||'y' as pj FROM sr_main
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1 kay
2 kay
3 kay
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2、Mysql版本：
2.1执行语句：SELECT concat( 'k','a') as pj FROM sr_main
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ka
ka
Ka
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2.2执行语句：SELECT concat( 'k','a','y') as pj FROM sr_main
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kay
kay
Kay
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2.3执行语句：SELECT 'k'||'a'||'y' as pj FROM sr_main
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0
0
0
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分析两者差异：
Oracle中可以用||拼接多个字符串，但用concat只能拼两个，否则报错
Mysql中可以用concat拼接多个，但用||无法拼接字符串，会显示零
综合两者：SELECT concat(concat( 'k','a'),'y') as pj FROM sr_main
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kay
kay
Kay
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此方法可以实现两者通用