SQL Server : Transpose rows to columns

One way to do it if tagID values are known upfront is to use conditional aggregation

SELECT TimeSeconds,
       COALESCE(MAX(CASE WHEN TagID = 'A1' THEN Value END), 'n/a') A1,
       COALESCE(MAX(CASE WHEN TagID = 'A2' THEN Value END), 'n/a') A2,
       COALESCE(MAX(CASE WHEN TagID = 'A3' THEN Value END), 'n/a') A3,
       COALESCE(MAX(CASE WHEN TagID = 'A4' THEN Value END), 'n/a') A4
  FROM table1
 GROUP BY TimeSeconds

or if you’re OK with NULL values instead of 'n/a'

SELECT TimeSeconds,
       MAX(CASE WHEN TagID = 'A1' THEN Value END) A1,
       MAX(CASE WHEN TagID = 'A2' THEN Value END) A2,
       MAX(CASE WHEN TagID = 'A3' THEN Value END) A3,
       MAX(CASE WHEN TagID = 'A4' THEN Value END) A4
  FROM table1
 GROUP BY TimeSeconds

or with PIVOT

SELECT TimeSeconds, A1, A2, A3, A4
  FROM
(
  SELECT TimeSeconds, TagID, Value
    FROM table1
) s
PIVOT
(
  MAX(Value) FOR TagID IN (A1, A2, A3, A4)
) p

Output (with NULLs):

TimeSeconds A1      A2     A3    A4
----------- ------- ------ ----- -----
1378700244  3.75    NULL   NULL  NULL
1378700245  30.00   NULL   NULL  NULL
1378700304  1.20    NULL   NULL  NULL
1378700305  NULL    56.00  NULL  NULL
1378700344  NULL    11.00  NULL  NULL
1378700345  NULL    NULL   0.53  NULL
1378700364  4.00    NULL   NULL  NULL
1378700365  14.50   NULL   NULL  NULL
1378700384  144.00  NULL   NULL  10.00

---

If you have to figure TagID values out dynamically then use dynamic SQL

DECLARE @cols NVARCHAR(MAX), @sql NVARCHAR(MAX)

SET @cols = STUFF((SELECT DISTINCT ',' + QUOTENAME(TagID)
            FROM Table1
            ORDER BY 1
            FOR XML PATH(''), TYPE
            ).value('.', 'NVARCHAR(MAX)'),1,1,'')

SET @sql="SELECT TimeSeconds, " + @cols + '
              FROM
            (
              SELECT TimeSeconds, TagID, Value
                FROM table1
            ) s
            PIVOT
            (
              MAX(Value) FOR TagID IN (' + @cols + ')
            ) p'

EXECUTE(@sql)