我有两张桌子，一张是以不同的间隔拉动美元的加密值（每差不多30分钟）。
'crypto'表
+-----+---------------------+-------------+-------------+-----------+
| id | date_hour | crypto_code | crypto_name | price_usd |
+-----+---------------------+-------------+-------------+-----------+
| 13 | 2018-05-24 16:43:29 | BTC | Bitcoin | 7553.75 |
| 14 | 2018-05-24 16:43:29 | ETH | Ethereum | 584.764 |
| 15 | 2018-05-24 16:43:29 | BTM | Bytom | 0.617043 |
| 16 | 2018-05-24 16:43:29 | DCR | Decred | 102.578 |
| 17 | 2018-05-24 16:43:29 | SC | Siacoin | 0.0164824 |
| 18 | 2018-05-24 16:43:29 | ZEC | Zcash | 289.619 |
| 19 | 2018-05-24 17:00:01 | BTC | Bitcoin | 7528.51 |
| 20 | 2018-05-24 17:00:01 | ETH | Ethereum | 584.769 |
| 21 | 2018-05-24 17:00:01 | BTM | Bytom | 0.614629 |
| 22 | 2018-05-24 17:00:01 | DCR | Decred | 102.229 |
| 23 | 2018-05-24 17:00:01 | SC | Siacoin | 0.016547 |
| 24 | 2018-05-24 17:00:01 | ZEC | Zcash | 286.252 |
| 25 | 2018-05-24 17:31:01 | BTC | Bitcoin | 7508.15 |
| 26 | 2018-05-24 17:31:01 | ETH | Ethereum | 584.024 |
| 27 | 2018-05-24 17:31:01 | BTM | Bytom | 0.607491 |
| 28 | 2018-05-24 17:31:01 | DCR | Decred | 101.801 |
| 29 | 2018-05-24 17:31:01 | SC | Siacoin | 0.0163432 |
| 30 | 2018-05-24 17:31:01 | ZEC | Zcash | 289.187 |
| 31 | 2018-05-24 18:00:01 | BTC | Bitcoin | 7545.02 |
| 32 | 2018-05-24 18:00:01 | ETH | Ethereum | 587.121 |
| 33 | 2018-05-24 18:00:01 | BTM | Bytom | 0.612734 |
| 34 | 2018-05-24 18:00:01 | DCR | Decred | 102.667 |
| 35 | 2018-05-24 18:00:01 | SC | Siacoin | 0.0165401 |
| 36 | 2018-05-24 18:00:01 | ZEC | Zcash | 289.686 |
| 37 | 2018-05-24 18:31:01 | BTC | Bitcoin | 7574.54 |
| 38 | 2018-05-24 18:31:01 | ETH | Ethereum | 590.538 |
| 39 | 2018-05-24 18:31:01 | BTM | Bytom | 0.620313 |
| 40 | 2018-05-24 18:31:01 | DCR | Decred | 103.306 |
| 41 | 2018-05-24 18:31:01 | SC | Siacoin | 0.0164874 |
| 42 | 2018-05-24 18:31:01 | ZEC | Zcash | 291.421 |
| 43 | 2018-05-24 19:00:02 | BTC | Bitcoin | 7600.11 |
| 44 | 2018-05-24 19:00:02 | ETH | Ethereum | 594.962 |
| 45 | 2018-05-24 19:00:02 | BTM | Bytom | 0.622085 |
| 46 | 2018-05-24 19:00:02 | DCR | Decred | 103.229 |
| 47 | 2018-05-24 19:00:02 | SC | Siacoin | 0.0166324 |
| 48 | 2018-05-24 19:00:02 | ZEC | Zcash | 296.651 |
| 49 | 2018-05-24 19:31:01 | BTC | Bitcoin | 7599.6 |
| 50 | 2018-05-24 19:31:01 | ETH | Ethereum | 593.13 |
| 51 | 2018-05-24 19:31:01 | BTM | Bytom | 0.620733 |
| 52 | 2018-05-24 19:31:01 | DCR | Decred | 101.368 |
| 53 | 2018-05-24 19:31:01 | SC | Siacoin | 0.0165951 |
| 54 | 2018-05-24 19:31:01 | ZEC | Zcash | 292.538 |
| 55 | 2018-05-24 20:00:01 | BTC | Bitcoin | 7603.72 |
另一个每X分钟监控几个钱包的人称为“生产”
+-----+----------+---------------------+--------------+
| id | currency | date_hour | bal_conf |
+-----+----------+---------------------+--------------+
| 1 | CMM | 2018-05-25 08:00:00 | 24.87 |
| 197 | ETH | 2018-05-24 10:56:53 | 0.82431267 |
| 198 | SIA | 2018-05-24 10:56:54 | 0.77711006 |
| 199 | BTM | 2018-05-24 10:58:02 | 81.33782690 |
| 201 | SIA | 2018-05-24 11:00:23 | 0.77711006 |
| 202 | ETH | 2018-05-24 11:30:15 | 0.83045272 |
| 203 | SIA | 2018-05-24 11:30:16 | 0.77711006 |
| 204 | ETH | 2018-05-24 12:00:23 | 0.83441691 |
| 205 | SIA | 2018-05-24 12:00:23 | 0.77711006 |
| 206 | ETH | 2018-05-24 12:30:18 | 0.84046355 |
| 207 | SIA | 2018-05-24 12:30:31 | 0.77711006 |
| 208 | ETH | 2018-05-24 13:00:14 | 0.84323421 |
| 209 | SIA | 2018-05-24 13:00:18 | 0.77711006 |
| 211 | SIA | 2018-05-24 13:30:18 | 0.77711006 |
| 212 | BTM | 2018-05-24 13:58:05 | 85.18699829 |
| 213 | BTM | 2018-05-24 14:00:01 | 85.18699829 |
| 215 | SIA | 2018-05-24 14:00:19 | 0.77711006 |
| 216 | BTM | 2018-05-24 14:02:02 | 85.18699829 |
| 217 | ETH | 2018-05-24 14:02:45 | 0.85096095 |
| 218 | SIA | 2018-05-24 14:02:54 | 0.77711006 |
| 219 | ETH | 2018-05-24 14:03:32 | 0.85096095 |
| 220 | SIA | 2018-05-24 14:03:36 | 0.77711006 |
| 221 | BTM | 2018-05-24 14:04:01 | 85.18699829 |
| 222 | BTM | 2018-05-24 14:06:01 | 85.41043305 |
| 223 | ETH | 2018-05-24 14:09:55 | 0.85096095 |
| 224 | SIA | 2018-05-24 14:10:17 | 0.77711006 |
| 225 | BTM | 2018-05-24 14:30:01 | 85.75066324 |
| 226 | ETH | 2018-05-24 14:30:10 | 0.85363614 |
| 227 | SIA | 2018-05-24 14:30:13 | 0.77711006 |
| 228 | BTM | 2018-05-24 15:00:01 | 86.67740965 |
| 229 | ETH | 2018-05-24 15:00:12 | 0.85694953 |
| 230 | SIA | 2018-05-24 15:00:14 | 0.77711006 |
| 231 | BTM | 2018-05-24 15:30:01 | 87.02525693 |
| 233 | SIA | 2018-05-24 15:30:18 | 0.77711006 |
| 234 | ETH | 2018-05-24 15:34:33 | 0.86264950 |
| 235 | SIA | 2018-05-24 15:34:40 | 0.77711006 |
| 236 | ETH | 2018-05-24 15:38:30 | 0.86264950 |
| 237 | SIA | 2018-05-24 15:38:31 | 0.77711006 |
| 239 | SIA | 2018-05-24 15:40:15 | 0.77711006 |
| 240 | BTM | 2018-05-24 15:44:01 | 87.49497772 |
| 241 | BTC | 2018-05-24 15:48:37 | 0.03155398 |
| 243 | SIA | 2018-05-24 15:50:20 | 0.77711006 |
| 244 | BTC | 2018-05-24 15:51:22 | 0.03155398 |
| 245 | BTM | 2018-05-24 15:55:01 | 87.67270990 |
| 246 | BTC | 2018-05-24 15:57:01 | 0.03155398 |
| 247 | BTC | 2018-05-24 16:00:02 | 0.03155398 |
| 248 | BTM | 2018-05-24 16:00:01 | 87.67270990 |
| 250 | SIA | 2018-05-24 16:00:21 | 0.77711006 |
| 251 | ETH | 2018-05-24 16:10:15 | 0.86810400 |
| 252 | SIA | 2018-05-24 16:10:19 | 0.77711006 |
| 253 | BTM | 2018-05-24 16:11:01 | 88.15004778 |
| 254 | BTC | 2018-05-24 16:19:01 | 0.03155398 |
| 256 | SIA | 2018-05-24 16:20:19 | 0.77711006 |
| 257 | BTM | 2018-05-24 16:22:01 | 88.15004778 |
我需要为生产表中的每个date_hour找到与相应的crypto_code（production.currency = crypto.currency）最接近的price_usd值（最接近时间）
我试过这段代码：
SELECT t.date_hour,t.currency,t.bal_conf, s.price_usd
FROM production t
INNER JOIN crypto s
ON(t.date_hour like s.date_hour and s.crypto_code=t.currency)
但是它没有返回所有的生产记录，因此我需要在加密表中没有完全匹配的日期，以找到最接近的日期。
答案 0 :(得分：1)
我会使用子查询执行此操作，从货币代码匹配的price_usd获取crypto，absolute value的difference of the time stamps等于差异的最小值货币代码的时间戳。我们在另一个子查询中获得最小值。
SELECT p.date_hour,
p.currency,
p.bal_conf,
(SELECT c.price_usd
FROM crypto c
WHERE c.crypto_code = p.currency
AND (SELECT min(abs(timestampdiff(second, ci.date_hour, p.date_hour)))
FROM crypto ci
WHERE ci.crypto_code = p.currency)
= abs(timestampdiff(second, c.date_hour, p.date_hour))
ORDER BY c.date_hour DESC
LIMIT 1) price_usd
FROM production p;
当ORDER BY c.date_hour DESC LIMIT 1的两个或多个时间戳产生与生产中给定时间戳相同的差异时，crypto只处理罕见事件。在这种情况下，使用最新的时间戳。
支持此查询的索引可能是
CREATE INDEX crypto_crypto_code_date_hour
ON crypto
(crypto_code,
date_hour);
表示最里面的子查询和
CREATE INDEX crypto_crypto_code_date_hour_price_usd
ON crypto
(crypto_code,
date_hour,
price_usd);
表示最外面的子查询。 （外部查询没有，因为production没有条件。）