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| 左/右外连接 |`SELECT * FROM a, b WHERE a.id = b.id(+);`<br/>`SELECT * FROM a, b WHERE a.id(+) = b.id;`|`SELECT * FROM a LEFT JOIN b ON a.id = b.id;`<br/>`SELECT * FROM a RIGHT JOIN b ON a.id = b.id;`| 关联查询时,TiDB 不支持使用 (+) 实现左/右关联,只能通过 `LEFT JOIN` 或 `RIGHT JOIN` 实现。|
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|`NVL()`|`NVL(key,val)`|`IFNULL(key,val)`| 如果该字段值为 `NULL`,则返回 val 值,否则返回该字段的值。 |
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|`NVL2()`|`NVL2(key, val1, val2)`|`IF(key is NOT NULL, val1, val2)`| 如果该字段值非 `NULL`,则返回 val1 值,否则返回 val2 值。|
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|`DECODE()`| <li>`DECODE(key,val1,val2,val3)`</li><li>`DECODE(value,if1,val1,if2,val2,...,ifn,valn,val)`</li> | <li>`IF(key=val1,val2,val3)`</li><li>`CASE WHEN value=if1 THEN val1 WHEN value=if2 THEN val2,...,WHEN value=ifn THEN valn ELSE val END`</li> | <li>如果该字段值等于 val1,则返回 val2,否则返回 val3。</li><li>当该字段值满足条件 1 (if1) 时,返回 val1,满足条件 2 (if2) 时,返回 val2,满足条件 3 (if3) 时,返回 val3。</li> |
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|`DECODE()`| <ul><li>`DECODE(key,val1,val2,val3)`</li><li>`DECODE(value,if1,val1,if2,val2,...,ifn,valn,val)`</li></ul>| <ul><li>`IF(key=val1,val2,val3)`</li><li>`CASE WHEN value=if1 THEN val1 WHEN value=if2 THEN val2,...,WHEN value=ifn THEN valn ELSE val END`</li></ul>| <ul><li>如果该字段值等于 val1,则返回 val2,否则返回 val3。</li><li>当该字段值满足条件 1 (if1) 时,返回 val1,满足条件 2 (if2) 时,返回 val2,满足条件 3 (if3) 时,返回 val3。</li></ul> |
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